How to pass a URL through a portal page

I've been searching for several days, including posting this question on another forum, but I have not found a good example of how to do this.
I have an XHTML page with multiple different links to a database driven ASP.NET 2.0 sub site that look something like this:
 
href="sales/default.aspx?category_id=657230A7-37AB-4A62-8C77-2B39A78BEF3E&ControlName=SubCategories"
 
I want to pass this URL to a validation page the first
time a user clicks one of these links in a session. On subsequent
clicks, I want to bypass the validation page and follow the link
directly.
I can't figure out how to pass the URL dynamically. I'm using the following validatio ...