Пример POST запроса:

var request = (HttpWebRequest) HttpWebRequest.Create("http://url");
request.Method = "POST";
const string command = @"Текст POST запроса";
byte[] bytes = Encoding.ASCII.GetBytes(command);
request.ContentLength = bytes.Length;
using (var stream = request.GetRequestStream())
{
    stream.Write(bytes, 0, bytes.Length);
}
using(var stream = new StreamReader(request.GetResponse().GetResponseStream()))
{
    Console.WriteLine(stream.ReadToEnd());
}