You can possibly POST your authentication information to the form, and then have the script redirect the user:

http://username:password@www.example.com/resource/
[download]

However, this puts the username and password not only in the clear for all to see, but in plain-text as part of the request.

Or, if your username/password is fairly static (never changes through use of the form), just have the form post its results to a CGI script using a variation of the URL above.