Well, you can only POST a message using a POST query. Any parameters passed via a URL, create a GET query.

Now, you could use JavaScript, to create a form, a lot of invisible variables, populate those variables, then call the form's "submit()" method, which will POST the form. You can only do that on a page you "own", of course.

"AJAX" is a funny little beast - it refers to scripting the XMLHttpRequest object. It doesn't do a POST, it fires off another request, "behind the scenes", so that the user doesn't see any page refresh.