public static string WebRequestPostData(string url)
        {
            HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);  
            request.ContentType = @"text/html; charset=windows-1251"; //
            request.Method = "POST";
           
            byte[] message = new byte[256];
            
            message = Encoding.GetEncoding(1251).GetBytes("абвгд");
            
            using (Stream requestStream = request.GetRequestStream())
            {
                requestStream.Write(message, 0, message.Length);
                requestStream.Close();
            }

            HttpWebResponse response = (HttpWebResponse)request.GetResponse();
            using (StreamReader sr = new StreamReader(response.GetResponseStream(), Encoding.GetEncoding(1251)))
            {
                var responseString = sr.ReadToEnd();
                return responseString;
            } 
        }