Get 3 random custom-textured Pokémon from the Genesis Chest. You can check the contents and use your keys at /warp chests.
Note that for all Legendary and Mythical Pokémon in the chest, you will receive a Paint Brush for that Pokémon, instead of the Pokémon directly. When you inspect the chest contents in-game, these are labeled with "Paint Brush" in the item name.