Hook on add to cart link

[adw]

I will be linking to my WHMCS site from a couple of other websites and want to pass a site=name parameter in the URL to set a cookie so that I can show the correct logo in the header:

add_hook('ClientAreaPage', 1, function () {
    // check $_GET['site'] and set cookie
});

However, this code does not work with the "Add to Cart" link such as: http://domain.com/cart.php?a=add&pid=1&site=name

This link redirects to the confproduct page and only then does the hook get fired, but then my site parameter is gone.

Is there a hook that will fire on the ?a=add link, before it gets redirected?

[adw]

Ok, I figured it out by using an in-between PHP script that will set the cookie and then redirect to cart.php