Oauth2 Yahoo Gemini API

I am having trouble calling the Yahoo Gemini API to access Yahoo Gemini Advertising from my application in the C # console (desktop).

Here are the steps I used:

• Create an installed application on https://developer.yahoo.com/apps/create/
  
  . It gave me both {Client ID}
  
  and {Client Secret}
  
  .
• https://api.login.yahoo.com/oauth2/request_auth?client_id={Client ID} &redirect_uri=oob&response_type=code&language=en-us
  
  ... This takes me to the Yahoo login screen where I login. Click the "Agree" button and the next screen will display a seven-letter authorization code (say, nzbcns9). I am writing this authorization code.

• Then I use the following code to try and get the access token:

  class Program
  {
  static void Main(string[] args)
  {
      string clientId = {Client ID};
      string secret = {Client Secret};
  
      var request = WebRequest.Create(@"https://api.login.yahoo.com/oauth2/get_token");
      request.Method = "POST";
  
      SetBasicAuthHeader(request, clientId, secret);
  
      string postData = "grant_type=authorization_code&redirect_uri=oob&code=nzbcns9";
      ASCIIEncoding encoding = new ASCIIEncoding();
      byte[] byte1 = encoding.GetBytes(postData);
      request.ContentLength = byte1.Length;
      Stream dataStream = request.GetRequestStream();
      dataStream.Write(byte1, 0, byte1.Length);
      dataStream.Close();
  
      request.ContentType = "application/x-www-form-urlencoded";
      var response = request.GetResponse();
      Console.WriteLine(((HttpWebResponse)response).StatusDescription);
  }
  
  static void SetBasicAuthHeader(WebRequest request, String userName, String userPassword)
  {
      string authInfo = userName + ":" + userPassword;
      authInfo = Convert.ToBase64String(Encoding.Default.GetBytes(authInfo));
      request.Headers["Authorization"] = "Basic " + authInfo;
  }
  }
  
        
  
          
          
          
        
  
      

Then I get

Unhandled Exception: System.Net.WebException: The remote server responded with an error: (401) Unauthorized. at System.Net.HttpWebRequest.GetResponse ().

What have I done wrong?

I am also trying to post the same post using Fiddler, I get

{"error": "invalid_request"}