C # using post json data submission

Try a day, try a variety of methods, about the most direct and convenient method. 

//地址
            string _url = "https://www.dXXXayup.ink/api/User/Login";
            //json参数
            string jsonParam = "{ phonenumber:\"18665885202\",pwd:\"tsp\"}";
            var request = (HttpWebRequest)WebRequest.Create(_url);
            request.Method = "POST";
            request.ContentType = "application/json;charset=UTF-8";
            byte[] byteData = Encoding.UTF8.GetBytes(jsonParam);
            int length = byteData.Length;
            request.ContentLength = length;
            Stream writer = request.GetRequestStream();
            writer.Write(byteData, 0, length);
            writer.Close();
            var response = (HttpWebResponse)request.GetResponse();
            var responseString = new StreamReader(response.GetResponseStream(), Encoding.GetEncoding("utf-8")).ReadToEnd();
            MessageBox.Show(responseString.ToString());