It is very easy to do, you can add script using source view of the editor in an Html Content Instance (or in layout.master) like this:
<script type="text/javascript">
$(document).ready(function() { alert('hello there'); });
</script>
you would just put your own code to show the popup where I have used alert function.
this uses jquery to call the function after the page has loaded.
Hope it helps,
Joe