Post Json HttpWebrequest in windows phone 8

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C#

Windows

When i try to send the JSON data to server it is throwing exception
is there change setting in Windows phone 8

here my code sample :-

private void PostJsonRequest() 
{ 
   //Please replace your webservice url here
   string AuthServiceUri = "http://test.postjson.com/Login?"; 
   HttpWebRequest spAuthReq = HttpWebRequest.Create(AuthServiceUri) as HttpWebRequest; 
   spAuthReq.ContentType = "application/json"; 
   spAuthReq.Method = "POST"; 
   spAuthReq.BeginGetRequestStream(new AsyncCallback(GetRequestStreamCallback), spAuthReq); 
} 
 
void GetRequestStreamCallback(IAsyncResult callbackResult) 
{ 
   HttpWebRequest myRequest = (HttpWebRequest)callbackResult.AsyncState; 
   Stream postStream = myRequest.EndGetRequestStream(callbackResult); 
   string postData = "{'userid': '2','username':'subbu'}"; 
   byte[] byteArray = Encoding.UTF8.GetBytes(postData); 
   postStream.Write(byteArray, 0, byteArray.Length); 
   postStream.Close(); 
   myRequest.BeginGetResponse(new AsyncCallback(GetResponsetStreamCallback), myRequest); 
} 
 
void GetResponsetStreamCallback(IAsyncResult callbackResult) 
{ 
   try 
   { 
      HttpWebRequest request = (HttpWebRequest)callbackResult.AsyncState; 
      HttpWebResponse response = (HttpWebResponse)request.EndGetResponse(callbackResult); 
      string responseString = ""; 
      Stream streamResponse = response.GetResponseStream(); 
      StreamReader reader = new StreamReader(streamResponse); 
      responseString = reader.ReadToEnd(); 
      streamResponse.Close(); 
      reader.Close(); 
      response.Close(); 
      string result = responseString; 
   } 
   catch (Exception e) 
   { 
 
   }
}

+       ex  {System.Net.WebException: The remote server returned an error: NotFound. ---> System.Net.WebException: The remote server returned an error: NotFound.
   at System.Net.Browser.ClientHttpWebRequest.InternalEndGetResponse(IAsyncResult asyncResult)
   at System.Net.Browser.ClientHttpWebRequest.<>c__DisplayClasse.<EndGetResponse>b__d(Object sendState)
   at System.Net.Browser.AsyncHelper.<>c__DisplayClass1.<BeginOnUI>b__0(Object sendState)
   --- End of inner exception stack trace ---
   at System.Net.Browser.AsyncHelper.BeginOnUI(SendOrPostCallback beginMethod, Object state)
   at System.Net.Browser.ClientHttpWebRequest.EndGetResponse(IAsyncResult asyncResult)
   at Track.View.StartTrac.GetResponsetStreamCallback(IAsyncResult callbackResult)} System.Exception {System.Net.WebException}