My client is using an ad tracking script in his shopping cart, so I need to put a variable into a string and send it off to the tracker. The variable is provided by the shopping cart via javascript, and I'm not sure how to put that into the string.

This is what I came up with, but I have no way of knowing if it works because I can't actually see the output.

Code:
<html>
<head>
<script>
    function orderNum() {
        document.write(ss_ordernum); // cart provided variable
    }
</script>

<script src="https://tags.mediaforge.com/js/xxx/?orderNumber=javascript:orderNum();">

</head>