Am getting the below error , can anyone give the solution

The remote server returned an error: (400) Bad Request.

var httpWebRequest = (HttpWebRequest)WebRequest.Create(" https://inventory.zoho.com/api/v1/contacts/contactpersons?authtoken=cb327707a0e8539254fcd3a58d896b43&organization_id=673578474");

            httpWebRequest.ContentType = "application/json;charset=UTF-8";

            httpWebRequest.Accept = "*/*";

            httpWebRequest.Method = "POST";

            httpWebRequest.Headers.Add("authtoken", "cb327707a0e8539254fcd3a58d896b43");

            httpWebRequest.Headers.Add("organization_id", "673578474");

            var vJsonData = "{\"contact_id\": 1523420000000070013,\"salutation\": \"Mr\",\"first_name\": \"Jams\",\"last_name\": \"jarald\",\"email\": \" jamsjarald@mobiusservies.in\",\"phone\": \"+044-123654\",\"mobile\": \"+91-9842326244\",\"skype\": \"skypePIM\",\"designation\": \"TESTING\",\"department\": \"TESTING Team\",\"enable_portal\": true }";

            var json = vJsonData.ToString();

            //json = "{" + json + "}";

            json = json.Replace(@"\", "");

            using (var writer123 = new System.IO.StreamWriter(httpWebRequest.GetRequestStream()))

            {

                writer123.Write(json);

                writer123.Flush();

                writer123.Close();

            }

            System.Net.ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls11 | SecurityProtocolType.Tls12;

            var httpResponse = (HttpWebResponse)httpWebRequest.GetResponse();

            var response123 = new HttpResponseMessage(HttpStatusCode.OK);

            using (var streamReader = new StreamReader(httpResponse.GetResponseStream()))

            {

                var objText = streamReader.ReadToEnd();

                var objJson = (Newtonsoft.Json.Linq.JObject)Newtonsoft.Json.JsonConvert.DeserializeObject(objText);

                response123.Content = new StringContent(objText);

            }