How to find out the FQDN of a domain?

Question

Active Directory

Please don't hit hard for a stupid question.
There is one old server on Windows Server 2003. The Domain Controller with Active Directory is spinning on it.
The challenge is to replace it. To do this, of course, you need to raise a secondary domain controller, and transfer rights from the first to the current one.
The Zentyal distro was chosen as the new controller.
In its settings there is an item with the FQDN of the main domain. Actually, how do you know?
The domain itself looks like this - aaa.thecompany.ru
But what to drive in the FQDN field is not entirely clear. The same? Or along with the name of the machine on which the old domain controller is deployed (bbb.aaa.thecompany.ru)?