You have a time until April 30th, 2022 to get Darkrai for Pokemon Brilliant Diamond and Pokemon Shining Pearl.
In order to secure Darkrai, you must first obtain “Member Card” item which is given to all players via “Mystery Gift” feature. Internet connection is required.
Once you get “Member Card“ you’ll have to fulfill specific conditions before adding Darkrai to your team. You will need to enter the “Hall of Fame” and obtain the National Pokedex.
After that, you can travel to “Newmoon Island” where you’ll find Darkrai.
Worth remembering that if you haven’t finished the game, you can obtain “Member Card” as soon as you enable “Mystery Gift” access. After that, once you enter “Half of Fame| you can obtain Darkrai at any time.
Have you befriended Darkrai or still trying to finish a game?