Attention, Trainers! If you made the rankings in the Pokémon Global Link Let’s Go! Cup Online Competition, you can now claim your participation prize. To do so, log in to the Pokémon Global Link using the game that you used to participate in the Let’s Go! Cup. A personal notification will appear with information on how to download a special mobile-sized wallpaper featuring the Pokémon you used in the Let’s Go! Cup. Accept the conditions in the personal notification, and you will be able to download your Pikachu or Eevee wallpaper. The Pikachu or Eevee wallpaper will only be available until September 27, 2018, at 23:59 UTC.
Source: Pokémon Global Link