Complication with 3.6.10

[FatalBreeze]

There a hack i built for my forum like a year ago.
The hack is a user agreement that users must accept before entering certain forums.
Because it's the same template for all those certain forum's id, in the template i wrote:

<form action="forumdisplay.php?f=$toForum" method="post">

And the the $toForum is set within a plugin ($toForum = $foruminfo['forumid']),
so that when the user clicks on "I Agree" he is redirected to the right forum.

I upgraded yesterday to 3.6.10 from 3.6.8 and now, when i use the hack, i get an error something about the security token, which is new since CSRF.

Now i now it's because of i'm redirecting back to the vbulletin scripts, i didn't quite understand what Kier wrote in vb.com.

How can i fix this? Thanks in advance.