Hi, I have no experience with HTTrack and websites requiring login credentials.
But you might be very much right, that the way you tried so far apparently
didn't work because HTTrack is "not logged in" properly, else you wouldn't end
up with the result you described.

Have a look at this example how login websites and HTTrack work together:
<http://httrack.kauler.com/help/CatchURL_tutorial>

Also you should be able to find some answers in the forum. As there is no
search function, what a pity, you may utilize the standard Google search with
the parameter to restrict it to this forum page and you should get results,
because I have the impression this is a question asked multiple times over the
years. So there should be hopefully helpful answers around on this forum.

Good luck!