Method to Post Json Data

                         private static string GetResponse(string requestData, string url)
                            {
                                string responseXML = string.Empty;
                                try
                            {
                                byte[] data = Encoding.UTF8.GetBytes(requestData);
                                HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);
                                request.Method = "POST";
                                request.ContentType = "application/json";
                                request.Headers.Add("Accept-Encoding", "gzip");
                                Stream dataStream = request.GetRequestStream();
                                dataStream.Write(data, 0, data.Length);
                                dataStream.Close();
                                WebResponse webResponse = request.GetResponse();
                                var rsp = webResponse.GetResponseStream();
                                if (rsp == null)
                            {
                             //throw exception
                            }
                                using (StreamReader readStream = new StreamReader(new GZipStream(rsp, CompressionMode.Decompress)))
                            {
                                responseXML = JsonConvert.DeserializeXmlNode(readStream.ReadToEnd()).InnerXml;
                            }
                             return responseXML;
                            }
                              catch (WebException webEx)
                            {
                                //get the response stream
                                WebResponse response = webEx.Response;
                                Stream stream = response.GetResponseStream();
                                String responseMessage = new StreamReader(stream).ReadToEnd();
                             }
                         }
                    

Important Note:
In case of 400 Bad Request, kindly verify the responseMessage in WebException.
You can use following link to validate your Json Request:- http://jsonlint.com/