Forget what I said. Is this what you are needing?
Check the Host Settings ...
Custom Login Redirect Url:
If login is disabled in the AppSettings (AllowLoginAndLogoff needs to be set to "false"), this is the URL users will be redirected to when they need to access the forum. Optionally add "" to the URL to pass the return URL to the custom Url. E.g. "http://mydomain.com/login.aspx?PreviousUrl={0}"