Automatically login to forum with site registered

I am developing a website using ASP classic. The website has its own SQL Server database where all the members are registered. I also intend to write into the forum database to store the same account details so they can be accessed by the forum.
 
What I require is to be able to allow members to log into the website I am devloping, but once logged in I want them to launch the forum.
 
My question is, what do I pass into the link to the forum to enable a particular account to be logged in so that the account holder can use the forum right away? I don't want them to sign into the forum again.
 
Thanks
Mike