Have you tried giving the Method and Content type?
clnt.Method = "POST";
clnt.ContentType = "application/x-www-form-urlencoded";
It will come like this :
HttpWebRequest httpWReq = (HttpWebRequest)WebRequest.Create(url);
httpWReq.Method = "POST";
httpWReq.ContentType = "application/x-www-form-urlencoded";
HttpWebResponse response = (HttpWebResponse)httpWReq.GetResponse();
string responseString = new StreamReader(response.GetResponseStream()).ReadToEnd();
return responseString;
Hope this helps you!