Either create a shortcut for the client (e.g. "http://(yoursite)/forum/forum.asp?FORUM_ID=1") with the ID number matching the forum which they want to view, or create a default page like below which redirects to that path and set that page in IIS as the entry page to the forum. Make sure that the "FORUM_ID" portion of the URL is capitalized.
Sample entry page ("index.htm")