Wcf with JSON input parameter

[OperationContract]
        [WebInvoke(UriTemplate = "/Login",
            RequestFormat = WebMessageFormat.Json,
            ResponseFormat = WebMessageFormat.Json, Method = "POST")]
        Login_Response Login(Login_Request Login_RequestP);

 HttpWebRequest TheRequestL = (HttpWebRequest)WebRequest.Create("URL");
                TheRequestL.ContentType = "application/json";
                TheRequestL.ContentLength = zInputJsonDataP.Length;

                TheRequestL.Method = "POST";
                TheRequestL.AllowAutoRedirect = false;

                Stream aRequestStreamL = TheRequestL.GetRequestStream();
                byte[] postBytes = System.Text.Encoding.ASCII.GetBytes(zInputDataP);
                aRequestStreamL.Write(postBytes, 0, postBytes.Length);
                aRequestStreamL.Close();

                HttpWebResponse aResponseL = (HttpWebResponse)TheRequestL.GetResponse();
                string zVersionL = aResponseL.ProtocolVersion.ToString();
                StreamReader aStreamReaderL = new StreamReader(aResponseL.GetResponseStream());

                StringBuilder aHttpCallXmlOutL = new StringBuilder();
                string zTempL;
                while ((zTempL = aStreamReaderL.ReadLine()) != null)
                {
                    aHttpCallXmlOutL.Append(zTempL);
                }

                aStreamReaderL.Close();
                zOutputDataP = aHttpCallXmlOutL.ToString();