Attention, Trainers! If you used Game Sync to connect your copy of Pokémon Ultra Sun or Pokémon Ultra Moon with the Pokémon Global Link between November 17, 2017, and January 30, 2018, you’re eligible to receive a free Master Ball as an introductory bonus. Now is the time to log in with your game to get your special code. Then, click here to redeem your code and claim your Master Ball. Remember, if you purchased both Pokémon Ultra Sun and Pokémon Ultra Moon and performed Game Sync with them both, you can earn a Master Ball for each game!
Source: Pokémon Global Link