string url = ConfigurationManager.AppSettings["ArcGIS_Server_URL"] + "/tokens/generateToken/?f=pjson";

            string data = string.Format("username={0}&password={1}&client=requestip&expiration={2}", username, password, ConfigurationManager.AppSettings["TokenExpirationTime"]);

            url = string.Format("{0}&{1}", url, data);

            // Create a new web request
            CustomWebRequest request = new CustomWebRequest(url, "POST");
            //Get Response
            string requestResult = request.GetResponse();

Generate Token request should not contain username and password in the query string