So, the portal must be installed to www.site.forum.ru

But how can I make the portal to be on www.site.ru?

Hallo.
Sorry for my English

I have such a structure:
Forum is on subdomen (site.forum.ru)
The domen (site.ru) is not used (empty).

I want the portal to be installed to site.ru

Russian support keeps silence on that question.
How can I do that correctly?
Thanks.