test.cs

[HttpGet]
[Route("AccessToken")]
public async System.Threading.Tasks.Task<ActionResult> AccessTokenAsync() {

  // security credentials
  string clientId = "";
  string clientSecret = "";

  string serviceUrl = "https://trial.dsserver.io";

  string ClientCredentials = "client_credentials";

  AccessTokenResponse token;
  HttpClient m_client = new HttpClient();

  // generate the payload
  var payload = new Dictionary<string, string> {
    ["grant_type"] = ClientCredentials,
  };

  // 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")
  };

  // Add basic auth header containing client id and secret
  string credentials = $"{clientId}:{clientSecret}";
  byte[] credentialsUtf8 = Encoding.UTF8.GetBytes(credentials);
  string credentialsB64 = Convert.ToBase64String(credentialsUtf8);
  tokenRequest.Headers.Authorization = 
    new AuthenticationHeaderValue("Basic", credentialsB64);

  // send the request
  var tokenResponse = await m_client.SendAsync(tokenRequest);

  // retrieve and return the token
  var tokenResponseStream = await tokenResponse.Content.ReadAsStringAsync();
  token = JsonConvert.DeserializeObject<AccessTokenResponse>(tokenResponseStream);

  return Ok(token);
}

public string UrlEncode(Dictionary<string, string> dict) {
  return string.Join("&", dict.Keys.Select(k => $"{k}={WebUtility.UrlEncode(dict[k])}"));
}