I kept trying to get an older version of HTTrack (not sure which version) to
work on an authenticated (password protected) area of a website. But, it kept
failing. After upgrading HTTrack to version 3.49.2, I got it to work using two
separate entries for the URL.

The first URL entry needed to be the one for the credential submission form
(the URL linked to by the button pressed after supplying the username and
password). Of course, this URL needs to include the credentials as a domain
prefix, as in:
  <https://username:password@mydomain.com/login>

I used the "Add URL" form to convert the special characters in my password to
URL-friendly codes, then changed http to https.

The second URL entry needed to be the one that contained all of the content I
needed to copy, as in:
  <https://mydomain.com/myauthenticatedcontent>