from <http://www.httrack.com/html/fcguide.html>:

 User Authentication Protocols

Smoe servers require user ID and password information in
order to gain access. In this example, the user ID smith
with password foobar is accessing www.all.net/private/index.html


httrack smith:foobar@www.all.net/private/index.html

For more advanced forms of authentication, such as those
involving forms and cookies of various sorts, an emerging
capability is being provided through th URL capture features
(--catchurl). This feature don't work all of the time.