Rest service throws a internal error 500 with C# code

HttpWebResponse response =(HttpWebResponse) request.GetResponse();

try
           {
               HttpWebRequest request =(HttpWebRequest) WebRequest.Create("https://smsmate.lk/api/sms/v2/send");
               request.Method = "POST";
               request.PreAuthenticate = true;

               request.Headers["Authorization"] = "ApiKey" + "XXXXXX-150a-4fae-9f43-ce6e49b92d4e";
               Mysms mymsg = new Mysms();
               mymsg.Message = "Test";
               mymsg.Addresses = "94719254547";
               // JsonConvert.DeserializeObject(mymsg);
               var postdata = "{" + "Message:" + mymsg.Message + "," + "Addresses:[" + mymsg.Addresses + "]}";
               byte[] byteArray = Encoding.UTF8.GetBytes(postdata);

               var data = Encoding.ASCII.GetBytes(postdata);

               //if (ContainsUnicodeCharacter(postdata))
               //{
               //    data = Encoding.UTF8.GetBytes(postdata);
               //}

               request.ContentType = "application/json";
               request.ContentLength = postdata.Length;
               Stream req = request.GetRequestStream();
               req.Write(byteArray, 0, byteArray.Length);
               req.Close();
               HttpWebResponse response =(HttpWebResponse) request.GetResponse();
               req = response.GetResponseStream();

           }
           catch (Exception ex)
           {
               throw ex;
           }