Windows Phone 8.1 and HTTP POST Request

Question

I'm trying to post data to a web server with such code:

HttpWebRequest request = WebRequest.Create(uri) as HttpWebRequest;

            //request.CookieContainer = new CookieContainer();
            //request.CookieContainer.Add(new Uri(uri), new Cookie());

            string postData = parameters.ToQueryString();
            byte[] byteArray = Encoding.UTF8.GetBytes(postData);

            request.Method = "POST";

            request.ContentType = "application/x-www-form-urlencoded";

            try
            {
                using (Stream dataStream = await request.GetRequestStreamAsync())
                {
                    dataStream.Write(byteArray, 0, byteArray.Length);
                }

                WebResponse response = await request.GetResponseAsync();

                using (Stream dataStream = response.GetResponseStream())
                {
                    StreamReader reader = new StreamReader(dataStream);
                    return await reader.ReadToEndAsync();
                }
            }
            catch (Exception e)
            {
                return e.Message + e.StackTrace;
            }

Those bits of information that I found on the Internet suggest that it's because response headers are incorrect, but it's not for sure. Could you please tell how to do http post request with parameters and if suggestion described above is correct, how to tell system.net not to check response headers?
This is how I'm calling this method:

 Dictionary<string, string> par = new Dictionary<string, string>();
            par.Add("station_id_from", "2218000");
            par.Add("station_id_till", "2200001");
            par.Add("train", "112Л");
            par.Add("coach_num", "4");
            par.Add("coach_class", "Б");
            par.Add("coach_type_id", "3");
            par.Add("date_dep", "1424531880");
            par.Add("change_scheme", "0");
            debugOutput.Text = await Requests.makeRequestAsync("http://booking.uz.gov.ua/purchase/coach", par);