Sending XMLHTTP Request In ASP.NET

I'm using XMLHTTP object to post a request from the client to the server without refreshing the client explorer.

my client code looks like this:

objXMLHTTP.open("POST", http://localhost/trySCTree/AIXH_BTree.aspx, false);
objXMLHTTP.setRequestHeader("Content-type", "text/xml");
objXMLHTTP.send("parm=123");

my server code looks like this:

<% string getParm = Request.Form["parm"]; %>

somehow I'm always getting a null at the getParm variable. why does the parameter is not sent right to the server ?