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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");

Instagram Basic Display API: RestSharp Invalid authorization code response

Trying to to add an Instagram feed to my ASP.NET C# web app using this and this documentation
The issue is that I always get the Invalid authorization code error message when trying to get the short lived token via RestSharp call to https://api.instagram.com/oauth/access_token
In Postman the response is successful, is there something wrong with the RestSharp API call?
RestSharp API call
var client = new RestClient(_shortLivedTokenRequestString);
var request = new RestRequest(Method.POST);
request.AlwaysMultipartFormData = true;
request.AddParameter("client_id", _Instagram_App_Id, ParameterType.RequestBody);
request.AddParameter("client_secret", _Instagram_App_Secret, ParameterType.RequestBody);
request.AddParameter("grant_type", "authorization_code", ParameterType.RequestBody);
request.AddParameter("redirect_uri", _Instagram_App_Redirect_Uri, ParameterType.RequestBody);
request.AddParameter("code", authorizationCode, ParameterType.RequestBody);
IRestResponse<GetShortLivedTokenResponse> response = client.Execute<GetShortLivedTokenResponse>(request);
response: {"error_type": "OAuthException", "code": 400, "error_message": "Invalid authorization code"}
response object
Postman request/response
Postman response headers

I recently had a similar problem that involved not removing the #_ added at the end of the code parameter like described here.
I don't know if thats your case but maybe it helps

My problem was that I did not remove the #_ as described in the instructions:

Why does this request works using HttpWebRequest but not with RestSharp?

I am consuming an API that expects a XML in the body request. First i consumed the api via Postman and it worked, then i used that tool of Postman to convert the request to RestCharp C# code, and then using that code the reponse that i was receiving was different compared to postman. After that, i used Fiddler to generate c# code with the postman request, and using that code that fiddler generated i was able to consume the API via code sucessfully. I am just trying to understand what is the difference between the code generated from postman and the code generated from Fiddler.
This is the code that is generated from Fiddler and it works:
HttpWebRequest request =
(HttpWebRequest)WebRequest.Create("http://x.x.x.x.x");
request.Accept = "*/*";
request.KeepAlive = true;
request.Method = "POST";
request.ServicePoint.Expect100Continue = false;
byte[] postBytes = System.Text.Encoding.UTF8.GetBytes(body);
request.ContentLength = postBytes.Length;
Stream stream = request.GetRequestStream();
stream.Write(postBytes, 0, postBytes.Length);
stream.Close();
response = (HttpWebResponse)request.GetResponse();
This is the code generated from Postman (slightly altered, but still the code that was generated from postman didn't work and i don't think that the changes that was made interfered with the result) using RestSharp that doesn't work:
var client = new RestClient("http://x.x.x.x.x");
client.ConfigureWebRequest((r) =>
{
r.ServicePoint.Expect100Continue = false;
r.KeepAlive = true;
});
var request = new RestRequest();
request.AddXmlBody(body);
IRestResponse response = client.Post(request);
return response;
I tried a lot of things in the RestSharp code, like adding a header with different content-types and encoding, for example
request.AddHeader("Content-Type", "text/xml;charset=utf-8");
but nothing worked. The response from the api when consumed by the RestSharp code says it got an error of NPE, which i believe it means NullPointerException, but since the api is working just fine via postman and the code generated by Fiddler, i don't think the problem is in the API. Btw, the parameter body in the code are the exact same in both codes.

It looks like the request body is not matching with expected content type by API. When the content does not match the content type expected by the API then you may get NPE error.
in your fiddler generated code you are sending XML string as text.
Please add the following code:
request.AddHeader("Content-Type", "text/plain");
request.AddParameter("undefined", "<YourXml></YourXml>", ParameterType.RequestBody);
or
request.AddHeader("Content-Type", "application/xml");
request.AddParameter("undefined", "<YourXml></YourXml>", ParameterType.RequestBody);
Instead of
request.AddXmlBody(body);

Postman to C#(Restsharp) code snippet issue

I am feeling pretty stupid right now I can not figure this out. I am using postman to generate some C#(Rest sharp) to post a file to a restAPI.
In postman it works file the code snippet that it generates for C# does not work. The response code shows an internal server error in C#. I can not figure out where the problem is.
Here is the code snippet.
client = new RestClient("https://login.mydgsi.ca/WebAPI/Attachment?aboutType=Candidate&referenceID=1314180%20&attachmentTypeID=Resume&Name=Resume&expirationDate=1900/01/01&Note=Candidate%20REeume");
request = new RestRequest(Method.POST);
request.AddHeader("Postman-Token", "b339e68d-9257-44eb-8698-1f3f0c86ebfe");
request.AddHeader("Cache-Control", "no-cache");
request.AddHeader("Authorization", "Bearer " + secruityToken.access_token);
request.AddHeader("content-type", "multipart/form-data; boundary=----WebKitFormBoundary7MA4YWxkTrZu0gW");
request.AddParameter("multipart/form-data; boundary=----WebKitFormBoundary7MA4YWxkTrZu0gW", "------WebKitFormBoundary7MA4YWxkTrZu0gW\r\nContent-Disposition: form-data; name=\"\"; filename=\"C:\\Users\\jbungay\\Documents\\MyJabberFiles\\ddemchuk#domain.not.set\\3vertical Number 1 Resume.docx\"\r\nContent-Type: application/vnd.openxmlformats-officedocument.wordprocessingml.document\r\n\r\n\r\n------WebKitFormBoundary7MA4YWxkTrZu0gW--", ParameterType.RequestBody);
IRestResponse response = client.Execute(request);
Here is fiddler shows on the postman post. I am new to Fiddler and Postman.
Screen of the inspector from Fidder

Try changing "Postman-Token" to just "token". This worked for me on another project. Keep in mind the request parameters change with every request.

RestSharp code generated form Postman does not work

I am testing the QuickPay API in Postman and everything works fine. When I use the Postman Code Generator to generate a RestSharp snippet of the succesful result, the code does not return a status 200 against the API, and nothing happens.
I have posted the generated code her (altered the auth code though)
var client = new RestClient("http://api.quickpay.net/subscriptions/18612/recurring?Accept-Version=v10&id=18427612&amount=9900&order_id=test1234");
var request = new RestRequest(Method.POST);
request.AddHeader("postman-token", "145e85d9-0e36-4e6a-2742-b36fb3dccadb");
request.AddHeader("cache-control", "no-cache");
request.AddHeader("accept-version", "v10");
request.AddHeader("authorization", "Basic OjczYWY3NzlmZDYzMjIxNGUx33yYj5266ZkO222Z322221M34334433434344ZWY=");
IRestResponse response = client.Execute(request);
Any help is very much appreciated.
Best regards

I found out that instead of posting parameters into the url as querystring parameters, I should post them as parameters into the reqeuest method. No need for the Postman token nor the cache-control headers. And importantly, the url should be https://api.quickpay.net/subscriptions/18612/recurring