A security token error while submitting form

[tmi]

I need to login from an external website, that similar to a single sign on system using vbulletin user account, at the moment i just included vbulletin file "login.php" into my external php script, and i can get all the session and user information.
The problem is when posting a form with that file included, it says "Your submission could not be processed because a security token was missing.", what is the best workaround to avoid this.

Thanks in advance