System.Net.WebException: "Удаленный сервер возвратил ошибку: (400) Недопустимый запрос

 var request = (HttpWebRequest)WebRequest.Create("https://dodopizza.ru/api/sendconfirmationcode");
            request.Method = "POST";
            request.ContentType = "application/x-www-form-urlencoded";
            request.ContentLength = 23;
            request.Accept = "*/*";
            request.Headers.Add(HttpRequestHeader.AcceptLanguage, "ru-RU,ru;q=0.8,en-US;q=0.5,en;q=0.3");
            request.Headers.Add(HttpRequestHeader.AcceptEncoding, "gzip, deflate, br");
            request.Referer = "https://dodopizza.ru/abakan/kirova101";
            request.Headers.Add("Origin", "https://dodopizza.ru");
            request.Host = "dodopizza.ru";
            request.UserAgent = "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:67.0) Gecko/20100101 Firefox/67.0";
            request.Headers.Add("X-Requested-With", "XMLHttpRequest");
            request.KeepAlive = true;
            byte[] content = Encoding.ASCII.GetBytes("phoneNumber=79157817206");
            request.ContentLength = content.Length;
 
            using (var _stream = request.GetRequestStream())
            {
                _stream.Write(content, 0, content.Length);
            }
 
            var postStream = (HttpWebResponse)request.GetResponse();
            var stream = postStream.GetResponseStream();
            var reader = new StreamReader(stream);