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Why is postman sending form data in an HTTP GET?

I received a Postman json collection from an API vendor that works perfectly, but has something mystifying to me: The request is in a GET format, yet there is an x-www-form-urlencoded body.
URL: https://login.microsoftonline.com/d1e<secret>9563/oauth2/token
And when I look at the postman-generated c# code, the mystery continues:
var client = new RestClient("https://login.microsoftonline.com/d1e...d3/oauth2/token");
client.Timeout = -1;
var request = new RestRequest(Method.GET);
request.AddHeader("Content-Type", "application/x-www-form-urlencoded");
request.AddParameter("grant_type", "client_credentials");
request.AddParameter("client_id", "c06bb...79");
request.AddParameter("client_secret", "7~u...D");
request.AddParameter("resource", "https://vault.azure.net");
IRestResponse response = client.Execute(request);
Console.WriteLine(response.Content);
Note the AddParameter constructions for a GET call. To me, this must be a slight-of-hand for merely adding those values to the querystring. But when I look at the postman console I see:
In the postman console I would have expected to see those params appended to the url as a querystring, and then everything would have made sense. But you can see that it's a bonafide Request Body.
When I make GET calls in my c# code I like to use the simple yet solid WebClient object to call the DownloadString() method. But this method is only for GETs and there's no way to send a form-post style body, understandably.
Is postman truly sending a GET with all those values being appended to the url as a querystring? And should I do the same in my DownloadString() call? Or is there something else going on here? Should I instead, in my c#, be calling the UploadString() method and sending a form post BODY as a GET??

Http protocol supports adding a body to a request, but the WebClient class you use doesn't. Presumably because it isn't considered the norm.
I'm sure there's good reasons for Microsoft using it in the OAuth flow though. Those guys normally do things right!
HTTP GET with request body

API is just an abstraction , you can send what ever you want to the API . It depends on the implementation , how the server handles these data.
Some services considers only what it requires and ignores other information
some services considers the entire requests and validates that it has only the allowed data. what should be allowed depends on the service
Postman is just a client that sends data to server , its upto you to decide what all information it should send . If you dont need any body then keep it as none. if you need some thing then add it.

Restsharp parameter behavior when setting ParameterType=GetOrPost

I face a weird intermittent issue with RestSharp. I use it to submit a POST authentication request to a server. But under certain conditions that I cannot isolate yet, this authentication call fails. By looking at the server log, I see a GET request instead of a POST. And I really wonder how the hell it is possible.
Here is the code to submit the authentication request :
var client = new RestClient(m_baseUrl);
var request = new RestRequest("https://dummyserver.com/api/auth", Method.POST);
request.AddParameter("client_id", apiCredentials.ApiKey, ParameterType.GetOrPost);
request.AddParameter("client_secret", apiCredentials.ApiSecret, ParameterType.GetOrPost);
request.AddHeader("Content-Type", "multipart/form-data");
IRestResponse response = await client.ExecutePostTaskAsync(request);
Since it is a distributed application, I don't have much info on the client request. Telemetry just confirms that the authentication failed.
Is it possible (known bug) that RestSharp transformed this request into a GET?
And, second question, is there any difference on the request being created with those two syntaxes:
request.AddParameter("client_id", apiCredentials.ApiKey, ParameterType.GetOrPost);
request.AddParameter("client_id", apiCredentials.ApiKey);
I need parameters to be submitted as form-data for security purposes.
Thanks for your help.

I don't think it's possible that a POST request gets executed as GET. Consider that you have some code (not the code from your question), which does that. Even using ExecutePost is redundant when you explicitly set the request type in the RestRequest constructor.
Concerning the second question, there's no real difference. The default parameter type is GetOrPost. I also believe that we use multipart/form-data for POST requests by default where there's no JSON or XML body.
You can easily find it by looking at the code of RestSharp.

Although weird, I've experienced a similar issue with you.
My solution is to use ExecuteAsPost():
var response = client.ExecuteAsPost(request, "POST");

Is there a way to retrieve the String the way it is actually uploaded to the server (as a whole)?

I am currently working on a OAuth2 implementation. However I am stuck on an Error 401. It seems like there is something wrong with my post request that is supposed to retrieve the access token from the Company the User logged in to. This is my code:
internal void RequestAccessToken(string code)
{
string requestBody = "grant_type="+ WebUtility.UrlEncode(GRANTTYPE)+ "&code=" + WebUtility.UrlEncode(code)+"&redirect_uri="+ WebUtility.UrlEncode(REDIRECT_URI);
WebClient client = new WebClient();
client.Headers.Add("Authorization",HeaderBase64Encode(CLIENT_ID, SECRETKEY));
var response = client.UploadString("https://thewebsiteiamcallingto.com/some/api", requestBody);
var responseString = client.OpenRead("https://thewebsiteiamcallingto.com/some/api");
}
My Questions are:
Is there anything wrong with the way I try to make the POST request ?
Is there a way to retrieve the whole string that is posted to the URI using UploadString?
P.S. I have seen this post regarding the POST creation. However I find the async part to be too complicated for my case.

Since we dont know the api documentation, I would suggest you to make a postman request and view the actual request sent and response received, and secondly make a request using your method and capture using a utility like wireshark and compare the difference.

upload feed to walmart

I am breaking my head trying to upload a feed to walmart, after many times trying i used postman to generate C# restsharp code for me, in postman it works, but when using the c# restsharp code it returns a mysterious error. like this:
"No message body writer has been found for response class FeedAcknowledgement"
what does that mean?
here is my code:
string requestUrl = "";
requestUrl = string.Format("https://marketplace.walmartapis.com/v2/feeds?feedType=inventory");
string method = "POST";
// string[] sig = getSig(method, requestUrl).Replace("\r", "").Split('\n');
var mySig = new Signature(ConsumerID, SecretKEY, requestUrl, method);
var s = mySig.TimeStamp;
var returendSigniture = mySig.GetSignature(s);
var client = new RestClient("https://marketplace.walmartapis.com/v2/feeds?feedType=inventory");
var request = new RestRequest(Method.POST);
//request.AddHeader("postman-token", "c325ba5f-813a-f990-7899-6bfc4b14aa1b");
request.AddHeader("cache-control", "no-cache");
request.AddHeader("content-type", "multipart/form-data; boundary=----WebKitFormBoundary7MA4YWxkTrZu0gW");
request.AddHeader("accept", "application/xml");
request.AddHeader("wm_consumer.id", "--");
request.AddHeader("wm_sec.auth_signature", returendSigniture);
request.AddHeader("wm_sec.timestamp", mySig.TimeStamp);
request.AddHeader("wm_qos.correlation_id", "123456abcdef");
request.AddHeader("wm_svc.name", "Walmart Marketplace");
request.AddParameter("multipart/form-data; boundary=----WebKitFormBoundary7MA4YWxkTrZu0gW", "------WebKitFormBoundary7MA4YWxkTrZu0gW\r\nContent-Disposition: form-data; name=\"BOUNDERY\"\r\n\r\n<?xml version=\"1.0\" encoding=\"utf-8\"?>\n<wm:inventory xmlns:wm=\"http://walmart.com/\">\n <wm:sku>PP00500-2PC</wm:sku>\n <wm:quantity>\n <wm:unit>EACH</wm:unit>\n <wm:amount>120</wm:amount>\n </wm:quantity>\n <wm:fulfillmentLagTime>1</wm:fulfillmentLagTime>\n</wm:inventory>\r\n------WebKitFormBoundary7MA4YWxkTrZu0gW--", ParameterType.RequestBody);
IRestResponse response = client.Execute(request);

I spent all day in figuring out how to request Walmart v3. I propose you the following two steps:
Use Walmart signer in order to generate signed token.
You will need to use HttpWebRequest for getting response from Walmart in a way similar to what is described here.

I have not been able to get this to work natively in C#, but I do have a work around.
The Java SDK can successfully submit multi-part requests to Walmart. I wrote a wrapper around the SDK functions that can accept basic command line input to read a text file and send the appropriate call with attached files. From here, you can just call the .jar file (I do it via dynamically generated batch file) from your C# program and receive responses back via text file. This is a sub-optimal system, but it works reliably and when the choice was between updating inventory on 2000 items every day and using some dirty code, I went with the Java wrapper method. This will be replaced as soon as the C# SDK comes out, but I believe this is one of the reasons why the C# SDK may be being delayed.
This solution was used, only after spending about a week trying to get boundaries / streams / attachments to work in C# and having zero success. Cases were also submitted to walmart and I was able to work with some of their top tier engineering support staff and this problem completely stumped them. I was able to trace the Java SDK execution all the way down to a built in Maven / Java function that constructed the web request so there's something under the hood that Java is doing with a multi-part request that isn't immediately clear in C#.

How to create an otrs ticket using a soap request

The lack of documentation on this subject coupled with the fact that I'm struggling with a learning curve on all fronts and making me really confused about where to start. I need to get this done using C# if possible. I apologize for the vagueness of this question, but I'm really lost. I would love links to comprehensive guides/references.
In my efforts to get this done, I've run into the following problems/questions:
I've created a web service using the otrs gui, with the operation CreateTicket, but requests via C# to my chosen namespace are returning 404 (not found). When I try to add a service reference or web reference with that namespace, I get the same error. However, when I plug that namespace into my browser as the url, it displays "customer.pl".
Can I send a soap request without adding the web service as a service reference in visual studio? Given the previous problem I'm having I can't do it that way. Would I just build the soap request string and write it to the web request's data stream with http://domain/rpc.pl as the uri?
If the answer to the previous question is yes... When trying the below code segment I get an internal server error (500) on the last line. However the header looks like a SOAP header which confuses me because I wouldn't have thought it got that far.
var document = new StringBuilder();
document.Append("<UserLogin>some user login</UserLogin>");
document.Append("<Password>some password</Password> ");
document.Append("<Ticket>");
document.Append("<Title>some title</Title> ");
document.Append("<CustomerUser>some customer user login</CustomerUser>");
document.Append("<Queue>some queue</Queue>");
document.Append("<State>some state</State>");
document.Append("<Priority>some priority</Priority>");
document.Append("</Ticket>");
document.Append("<Article>");
document.Append("<Subject>some subject</Subject>");
document.Append("<Body>some body</Body>");
document.Append("<ContentType>text/plain; charset=utf8</ContentType>");
document.Append("</Article>");
//var uri = new Uri("http://domain/injest");
var uri = new Uri("http://domain/rpc.pl");
var httpWebReq = (HttpWebRequest)WebRequest.Create(uri);
var bytePostData = Encoding.UTF8.GetBytes(document.ToString());
httpWebReq.Timeout = 5 * 1000;
httpWebReq.Method = "POST";
httpWebReq.ContentLength = bytePostData.Length;
httpWebReq.ContentType = "text/xml;charset=utf-8";
//httpWebReq.TransferEncoding=
//httpWebReq.ContentType = "application/xml";
//httpWebReq.Accept = "application/xml";
var dataStream = httpWebReq.GetRequestStream();
dataStream.Write(bytePostData, 0, bytePostData.Length);
dataStream.Close();
var httpWebResponse = (HttpWebResponse)httpWebReq.GetResponse();
Even if all you can offer is where to start, it would help me to know how to proceed, as I'm stumped.

You're using the rpc.pl endpoint which is part of the 'old' RPC-style interface.
You mention you added the web service via the GUI which means you're using the 'new' Generic Interface, which is indeed much easier from .Net.
The address of the endpoint is /otrs/nph-genericinterface.pl/Webservice/GenericTicketConnector or whatever you have called the web service in the admin section.