I've tried two approaches.

1. Already being logged in the forum (my own) and using the top forum page as
start URL.

2. Using the “landing” page, i.e., the ***.com page as start URL and
giving HTTrack my name and password.

Both approaches results in getting directed to the “real” webpage on the
net, and the content of the forum is never scanned.

The forum is a very old version of SMF.