How to use callback url

private string CreateToken(string message, string secret)
       {
           secret = secret ?? "";
           var encoding = new System.Text.ASCIIEncoding();
           byte[] keyByte = encoding.GetBytes(secret);
           byte[] messageBytes = encoding.GetBytes(message);
           using (var hmacsha256 = new HMACSHA256(keyByte))
           {
               byte[] hashmessage = hmacsha256.ComputeHash(messageBytes);
               return Convert.ToBase64String(hashmessage);
           }
       }

string dataUrl = "{\"url \": \"http://185.74.157.252:8096/json.aspx \",\"signaturePassword\" : \""+ signPassword + "\",\"test\":\"true\"}";

string Str = CallBackUrl("https://wwrm.workwave.com/api/v1/callback", dataUrl, userid, ApiKey);

string signPassword = CreateToken("http://185.74.157.252:8096/json.aspx" + " w/", ApiKey);

string CallBackUrl(string url, string urlregister, string bigApiUserID, string BigApiKey)
       {
           string responseStr = "";
           HttpStatusCode statusCode;
           WebRequest request = WebRequest.Create(url);
           request.Credentials = new NetworkCredential(bigApiUserID, BigApiKey);
           request.Method = "POST";
           byte[] bytes;
           bytes = System.Text.Encoding.ASCII.GetBytes(urlregister);
           request.Headers.Add("X-WorkWave-Key", BigApiKey);
           request.ContentType = "application/json";
           request.ContentLength = bytes.Length;
           Stream requestStream = request.GetRequestStream();
           requestStream.Write(bytes, 0, bytes.Length);
           requestStream.Close();
           HttpWebResponse response;
           try
           {
               response = (HttpWebResponse)request.GetResponse();
               statusCode = response.StatusCode;
               if (Convert.ToString(response.StatusCode) == "OK")
               {
                   Stream responseStream = response.GetResponseStream();
                   responseStr = new StreamReader(responseStream).ReadToEnd();
                   return responseStr;
               }
           }
           catch (WebException  ex)
           {
               using (var stream = ex.Response.GetResponseStream())
               using (var reader = new StreamReader(stream))
               {
                   string str = Convert.ToString(reader.ReadToEnd());
               }

           }


           return responseStr;
       }