Usually the UBB is installed in a subdirectory of the webdir
for example
httpdocs/ubb/

To Run the forum Portal you must redirect the users at http://www.domain.tld/ubb/ubbthreads.php

This does not look very friendly to the fact that most forum domains do run only the forum.
Is there a way to have the PORTAL webpage being installed at the webdir (httpdocs) and accessible from http://www.domain.tld/index.php directly? (and having the forum at some subdir ex. ubb/)

You mean like this?

http://www.ubbdev.com/

If so, check this topic:

http://www.ubbdev.com/forums/ubbthreads.php/ubb/showflat/Number/312683

What you are asking for is about 1/3 the way down the page.

That is exactly what I needed, Thank you!

forgive me if i'm missing something, but why not just install the forum in the web root if theres nothing but a forum there?