GetRequestStream()を使う
HttpWebRequest HttpWReq =
(HttpWebRequest)WebRequest.Create("http:\\domain.com\page.asp");
ASCIIEncoding encoding=new ASCIIEncoding();
string stringData = ""; //place body here
byte[] data = encoding.GetBytes(stringData);
HttpWReq.Method = "PUT";
HttpWReq.ContentType = ""; //place MIME type here
HttpWReq.ContentLength = data.Length;
Stream newStream = HttpWReq.GetRequestStream();
newStream.Write(data,0,data.Length);
newStream.Close();ネタ元