Pre-Populate hidden field with referral information / the previous URL

Hey there!

I am busy using the forms and would love to have a destinct feature:

I want to have a kind of "referral-system" incorporated with my order-form. This means essentially, that I want to know which domain/URL the User came from before filling out my form. I know this can be done with GA & goals similarly, but I am looking for a more convenient way, giving me all the information upon reading the (future) client's request.

I already tried a little jQuery-script (see http://www.gravityhelp.com/forums/topic/previous-url-post-into-field) but either it is "printed" into the first visible field (input:eq(0) - I tried admin-only without any luck) or not working (when using the CSS-workaround, both admin-only and hidden).

Looking forward to some insights here. Thanks in advance!

For reference, this is my CSS-edit (placed in header.php) - maybe there is an error and it would all work out the way I intended?

<script>
jQuery(document).ready(function() {
    var referral = "<?= urldecode($_GET['referral']); ?>";
    jQuery('.gform_wrapper .referral').val(referral);
});
</script>