สอบถาม Error c# ของการใช้ http client post *****************************

using (var client = new HttpClient())
                {
                    string url = System.Configuration.ConfigurationManager.AppSettings["URL_IAuth"];
                    HttpWebRequest reqHttp = HttpWebRequest.CreateHttp(url);
                    HttpRequestMessage requestMessage = new HttpRequestMessage(HttpMethod.Post, new Uri(url));
                    requestMessage.Headers.Add("apikey", "jSDQ0oGzzXeNcrw7rWUMAFbyJ0cIesCY");
                    //Content
                    var json = Newtonsoft.Json.JsonConvert.SerializeObject(request);
                    var content = new System.Net.Http.StringContent(json.ToString(), System.Text.Encoding.UTF8, "application/json");
                    requestMessage.Content = content;

                    Functions.log.Info("Post IAuth => " + json);

                    //Response
                    var response = client.SendAsync(requestMessage).Result;

                    using (HttpContent ctn = response.Content)
                    {
                        var r = ctn.ReadAsStringAsync();
                        res = JsonConvert.DeserializeObject<Response>(r.Result);
                    }
                }