test.cs

[HttpGet]
public string Token()
{
   string code = Request.Query["code"];

   // Security credentials
   string clientId = "";
   string clientSecret = "";
   string serviceUrl = "https://trial.dsserver.io";
   string authorizationCode = "authorization_code";

   BearerToken token;
   using (HttpClient client = new HttpClient())
   {
       // Generate the payload
       var payload = new Dictionary<string, string>
       {
           ["grant_type"] = authorizationCode,
           ["code"] = code,
           ["client_id"] = clientId,
           ["client_secret"] = clientSecret,
           ["redirect_uri"] = "https://localhost:7194/Home/Token",
       };

       // Token endpoint
       string requestUri = $"{serviceUrl}/oauth/token";

       // Create the request message
       var tokenRequest = new HttpRequestMessage(HttpMethod.Post, requestUri)
       {
           Content = new StringContent(
               UrlEncode(payload),
               Encoding.UTF8,
               "application/x-www-form-urlencoded")
       };

       // Send the request
       var tokenResponse = client.SendAsync(tokenRequest).Result;

       // Retrieve and return the token
       var tokenResponseStream = tokenResponse.Content.ReadAsStringAsync().Result;
       token = JsonConvert.DeserializeObject<BearerToken>(tokenResponseStream);
   }

   return token.AccessToken;
}