Use xmlhttp method and post it to your asp.net page:
<%@language=JScript%>
<%
var objSrvHTTP;
var objXMLDocument;
objSrvHTTP = Server.CreateObject ("Msxml2.ServerXMLHTTP.6.0");
objXMLDocument = Server.CreateObject ("Msxml2.DOMDocument.6.0");
objXMLDocument.async= false;
objXMLDocument.loadXML ("<msg><id>MyValueToTransfer</id></msg>");
objSrvHTTP.open ("POST","http://someotherserver/PostHandler.aspx",false);
objSrvHTTP.send (objXMLDocument);
Response.ContentType = "text/xml";
Response.Write (objSrvHTTP.responseXML.xml);
%>
Then you handle posted data from aspx. Also take care to select proper xmlhttp based on browser. You get to know the stuff from google.