vBulletin redirect after login

Facts:

- main site on www.DOMAIN.com

- vBulletin installed on forum.DOMAIN.com

- vBulletin authentication integrated in main site (n开发者_如何学Co problems here)

Question:

How can I redirect user to some URL from the www subdomain upon authentication?

Extra info:

I am already using the extra "url" field in the login form, but it only works for URLs relative to forum root (i.e. I can set it to "forum.php?...", but if I set it to "http://some_domain/" it is ignored and user is redirected to forum home after authentication).

Thanks,

Puiu