An authenticator app is an app that you install on your mobile device that generates a unique code to prove to a certain system that you are the owner of an account.
Enter the email address of the person to whom you would like to send the gift.
So far, some of the revealed skins are Superman and Rick Sanchez from Rick and Morty, and if it's anything like the previous seasons, there are to be some more unexpected reveals from Epic Games.
The exact process can differ with the methods used, though.
This means anyone without that device will be unable to log in, even if they have managed to acquire your details.
There is no other way to receive the Boogie Down emote in the game.