How to make a domain the main one?

Question

Search engines

There are 2 sites of the same office, one is old on the .RU domain, and the second is new on the .rf domain, which is old, indexed by search engines as the main (official), I need to do the opposite. How to be, men? Help, something needs to be done urgently.

Answer 1

Write in the file /robots.txt:
User-agent: *
Host: your_site.rf
Then all search engines will understand which mirror is the main one.

Answer 2

In the robots.txt file, but in the form of domain.rf, is this normal or is it necessary in the form iegdbjvko9c.xn--p1ai ?