Thanks Gustaf,
It turned out that I change the approach and wrote another script. I've added a form, and the variable is within a hidden input. Instead of an argument in the link, which now is the submit form button and holds another id.
I guess those 3 lines below explain much better what I've written above.
<form method="post" name="request_proposal" action="request-proposal-2" >
<input type="hidden" name="product_id" id="product_id" value="@product_id;noquote@" >
<include src="/packages/evex-event/lib/events" >
...