The Magic Arena (or MTGA) Leaderboard works a little different than the other ones. You enter by registering on arena.gentrymagic.com for free. Every week you will get an assigned opponent to play a match. All communication happens through the discord that is linked on that page as well. It is currently mainly in Dutch, but you should have no problems communicating in English.
Go to arena.gentrymagic.com/standings for the MTGA standings