Вот пример без обработки ошибок (сделай сам :)):
HttpWebRequest request = (HttpWebRequest)WebRequest.Create(targetUri);
string sendString = formParameterName + "=" + HttpUtility.UrlEncode(xmlData);
byte[] byteStream;
byteStream = System.Text.Encoding.UTF8.GetBytes(sendString);
request.Method = POST;
request.ContentType = "application/x-www-form-urlencoded";
request.ContentLength = byteStream.LongLength;
using(Stream writer = request.GetRequestStream())
{
writer.Write(byteStream, 0, (int)request.ContentLength);
writer.Flush();
}
HttpWebResponse resp = (HttpWebResponse)request.GetResponse();
//read the response