All Pokémon Sun and Moon players who made the rankings in the 2017 International Challenge February Online Competition can now claim their participation prize – a special code that can be redeemed for a Mawilite and a Beedrillite Mega Stone now through March 31, 2017, at 23:59 UTC. Have your Mawile or Beedrill hold these items to allow them to Mega Evolve in battle!
Just log in to the Pokémon Global Link (PGL) using the game that you used to participate in the 2017 International Challenge February 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 copy of Pokémon Sun or Pokémon Moon before March 31, 2017, at 23:59 UTC.
Source: Pokémon Global Link