If you made the rankings in the 2018 International Challenge March Online Competition, you can claim your participation prize of 50 Battle Points now. Log in to the Pokémon Global Link (PGL) using the game that you used to participate in the 2018 International Challenge March, and then click here. To redeem the code in your game, select Mystery Gift from the main menu, then select Receive Gift, select Get with Code/Password, and then follow the prompts to enter your special code in your copy of Pokémon Ultra Sun or Pokémon Ultra Moon.
Visit the Online Competitions page for 2018 International Challenge March Trainer and Pokémon rankings.
Source: Pokémon Global Link