钉钉开发中post异步调用问题

{"errcode":43002,"errmsg":"需要POST请求"}

/// <summary>
        /// 
        /// </summary>
        /// <param name="postUrl">post地址</param>
        /// <param name="paramData">参数</param>
        /// <param name="dataEncode">数据格式</param>
        /// <returns></returns>
        public static string HttpPost(string postUrl, string paramData, Encoding dataEncode)
        {
            string ret = string.Empty;
            try
            {
                byte[] byteArray = dataEncode.GetBytes(paramData); //转化
                HttpWebRequest webReq = (HttpWebRequest)WebRequest.Create(new Uri(postUrl));
                webReq.Method = "POST";
                webReq.ContentType = "application/json; charset=utf-8";

                webReq.ContentLength = byteArray.Length;
                Stream newStream = webReq.GetRequestStream();
                newStream.Write(byteArray, 0, byteArray.Length);//写入参数
                newStream.Close();
                HttpWebResponse response = (HttpWebResponse)webReq.GetResponse();
                StreamReader sr = new StreamReader(response.GetResponseStream(), dataEncode);
                ret = sr.ReadToEnd();
                sr.Close();
                response.Close();
                newStream.Close();
            }
            catch (Exception ex)
            {
                return ex.Message;
            }
            return ret;
        }