I have tried that and nothing happened at all.
I decided to think about it some more, to see how it all works. I first tried a hidden form field and used its ID in the onload, but I got error as it wasn't a select. So I thought I would create a select (later to be hidden by CSS) that only has one option set, the one from the URL querystring. This kinda worked, it populates the lists, but it doesn't show the correct selected one, it only show the first option, which is select a category.
What we need is an additional function that is only used on page load, and sets the selected one as well. It could grab the values from hidden form fields, or directly from the querystring if JS can do that.