If you registered for the Mega Melee Online Competition and completed at least one match, you can now claim your participation prize of 50 Battle Points. Just log in to the Pokémon Global Link (PGL) using the game that you used to participate in the Mega Melee competition and 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 Sun or Pokémon Moon game before October 30, 2017, at 23:59 UTC. Visit the Online Competitions page for Mega Melee competition Trainer and Pokémon rankings.
Source: Pokémon Global Link
Pokémon Blog 