We are not suggesting putting usernames of the old admins there. You don’t remember their passwords anyway. What the suggest is to put a username of ANY simple user here, so this user will become an admin and will be able to login to Admin Console. Or you don’t know any password of any of the users on your server? In that case this is a dead end. Well, it may be possible to edit the database and change it there if the password is not encrypted. But that’s dark waters for me.
You only have to put a username there. And then after starting Openfire, go to Admin Console and put same username into Login field and then put a password into Password field.