C#模拟传递参数为json格式的POST提交

今天在开发过程中遇到一个问题，在用PostMan测试调用另一个公司的接口时一直不成功，然后经查看，发现该接口需要传递json格式的值。

C#模拟提交代码如下：

        public ActionResult GetCarLocation(string vin)
        {
            RequestModel model = new RequestModel() { pageNum=0, pageSize=0, vinNum=vin };
            string param= JsonConvert.SerializeObject(model);
            string returnstr= PostMoths("http://188.88.88.88:8081/third/carinfo/list", param);
            return Content(returnstr) ;
        }

        public static string PostMoths(string url, string param)
        {
            string strURL = url;
            System.Net.HttpWebRequest request;
            request = (System.Net.HttpWebRequest)WebRequest.Create(strURL);
            request.Method = "POST";
            request.ContentType = "application/json;charset=UTF-8";
            string paraUrlCoded = param;
            byte[] payload;
            payload = System.Text.Encoding.UTF8.GetBytes(paraUrlCoded);
            request.ContentLength = payload.Length;
            Stream writer = request.GetRequestStream();
            writer.Write(payload, 0, payload.Length);
            writer.Close();
            System.Net.HttpWebResponse response;
            response = (System.Net.HttpWebResponse)request.GetResponse();
            System.IO.Stream s;
            s = response.GetResponseStream();
            string StrDate = "";
            string strValue = "";
            StreamReader Reader = new StreamReader(s, Encoding.UTF8);
            while ((StrDate = Reader.ReadLine()) != null)
            {
                strValue += StrDate + "\r\n";
            }
            return strValue;
        }

问题解决。