If you want to POST your Xml data using a named form parameter you need to do something like this:
HttpWebRequest request = HttpWebRequest.Create("http://yourdomain.com/whatever") as HttpWebRequest;
request.Method = "POST";
request.ContentType = "application/x-www-form-urlencoded";
Encoding e = Encoding.GetEncoding("iso-8859-1");
XmlDocument doc = new XmlDocument();
doc.LoadXml("<foo><bar>baz</bar></foo>");
string rawXml = doc.OuterXml;
// you need to encode your Xml before you assign it to your parameter
// the POST parameter name is myxmldata
string requestText = string.Format("myxmldata={0}", HttpUtility.UrlEncode(rawXml, e));
Stream requestStream = request.GetRequestStream();
StreamWriter requestWriter = new StreamWriter(requestStream, e);
requestWriter.Write(requestText);
requestWriter.Close();
web-servicesis a little misleading here. You're not consuming a WebService but rather posting Xml data like Form data.