click on the little mail icon next to your username in the top right hand corner. when it brings you to dms/"Memechat", click on the word "Memechat" and then the cat will appear :)
NOTE: Pokémon Sword or Pokémon Shield are REQUIRED to get the quest. If you don't have save data from either, you cannot obtain Shaymin in Legends Arceus.
It spawns in the Obsidian Fieldlands. There's a special request you get from Laventon's blackboard about giving thanks and it leads you to the Floaro Gardens area in the Fieldlands. When you're there you have a brief cutscene and then Shaymin appears. After the quest completes, you can catch Shaymin. Not too hard, thankfully.
