Вы можете использовать System.Net.HttpWebRequest :
Запрос
HttpWebRequest request= (HttpWebRequest)WebRequest.Create(url);
request.ContentType="application/x-www-form-urlencoded";
request.Method = "POST";
request.KeepAlive = true;
using (Stream requestStream = request.GetRequestStream())
{
requestStream.Write(BytePost,0,BytePost.Length);
requestStream.Close();
}
Ответ
HttpWebResponse response = (HttpWebResponse)request.GetResponse();
using(StreamReader sr = new StreamReader(response.GetResponseStream()))
{
responseString = sr.ReadToEnd();
}