Loading scripts after page load?

So, there's no way that this works:

window.onload = function(){
  <script language="JavaScript" src="http://jact.atdmt.com/jaction/JavaScriptTest"></script>
};

You can't freely drop HTML into the middle of javascript.

---

If you have jQuery, you can just use:

$.getScript("http://jact.atdmt.com/jaction/JavaScriptTest")

whenever you want. If you want to make sure the document has finished loading, you can do this:

$(document).ready(function() {
    $.getScript("http://jact.atdmt.com/jaction/JavaScriptTest");
});

---

In plain javascript, you can load a script dynamically at any time you want to like this:

var tag = document.createElement("script");
tag.src = "http://jact.atdmt.com/jaction/JavaScriptTest";
document.getElementsByTagName("head")[0].appendChild(tag);

The second approach is right to execute JavaScript code after the page has finished loading - but you don't actually execute JavaScript code there, you inserted plain HTML.
The first thing works, but loads the JavaScript immediately and clears the page (so your tag will be there - but nothing else).
(Plus: language="javascript" has been deprecated for years, use type="text/javascript" instead!)

To get that working, you have to use the DOM manipulating methods included in JavaScript. Basically you'll need something like this:

var scriptElement=document.createElement('script');
scriptElement.type = 'text/javascript';
scriptElement.src = filename;
document.head.appendChild(scriptElement);