OAuth 2.0 authentication in RestSharp - authentication

Question

I am trying to authenticate RESTful service (sabre REST api) using RESTsharp library but i am not able to authenticate it. I am using my Client id and secret. Please tell me how to authenticate using oAuth 2.0 authenticator.
I have tried this code. ( sabre is using OAuth 2.0 authentication )
public ActionResult Index()
{
var client = new RestClient("https://api.test.sabre.com");
client.Authenticator = new HttpBasicAuthenticator("myclientid", "myclientsecret");
RestRequest request = new RestRequest("/v1/auth/token", Method.POST);
request.AddHeader("Authorization", "Basic " + client);
request.AddHeader("Content-Type", "application/x-www-form-urlencoded");
request.AddParameter("grant_type", "client_credentials");
IRestResponse response = client.Execute(request);
var content = response.Content;
ViewBag.R = content;
return View();
}
i got this result
{"error":"invalid_client","error_description":"Credentials are missing or the syntax is not correct"}
please tell what i am doing wrong.
Thanks
Snapshot of Fiddler Comparison of Running code (not with RestSharp) and code using RestSharp is shown
With RestSharp

Answer 1

Seems to me like you are adding the Authorization header twice. The documentation here says
The authenticator’s Authenticate method is the very first thing called
upon calling RestClient.Execute
Looking at the implementation of HttpBasicAuthenticator, the Authenticate method adds the appropriate header to the request.
So remove the following line from your example:
request.AddHeader("Authorization", "Basic " + client);

Answer 2

You need to first obtain access token from Sabre that you can later use while making rest api calls.
The access token POST request looks like this:
POST https://api.test.sabre.com/v2/auth/token
Authorization: Basic ZVc5MWNtTnNhV1Z1ZEdsazplVzkxY21Oc2FXVnVkSE5sWTNKbGRBPT0=
Content-Type: application/x-www-form-urlencoded
grant_type=client_credentials
where the value of Authorization after Basic is the Base64 encoded string based on your clientId and secret
Refer to Sabre Authentication on how this string is created
So, in order to get the access token you just need to send a POST request with required header and request parameters and you do not need to use the Authenticator