This is a global Mystery Gift event and is available for only two days for all Pokémon Sword and Shield players. TRA1NERSCUP is the code to receive Shiny Amoonguss, which can now be redeemed until tomorrow, August 10 at 14:59 UTC. The Shiny Amoonguss on offer is a level 50 Pokémon and is equipped with the moves Spore, Clear Smog, Rage Powder and Protect.
If you want many of these, you can create new users on your Switch and start new games on each of them. Advance the game enough until you have Mystery Gift available on the game menu and claim the code on each file. Use the free Pokemon Home app on the Switch to consolidate them onto your main file.
Thanks, got mine!