If you registered for the Ultra Spooky Cup Online Competition and completed at least three matches, you can now claim your participation prize: a Shiny Mimikyu! Just log in to the PGL using the game you used to participate in the Ultra Spooky Cup Online Competition, and then click here. To redeem the code in your game, select Mystery Gift from the main menu, then select Receive Gift, and then select Get with Code/Password and follow the prompts to enter your special code. Be sure to enter the code in your Pokémon Ultra Sun or Pokémon Ultra Moon game before December 30, 2018 (00:00 UTC). This Shiny Mimikyu is a level 50 Pokémon with the moves Destiny, Mimic, Snatch and Trick. It is also holding a Rare Candy.
Source: Pokémon Global Link