The official Pokémon website has announced that from October 16th, 2022 until November 18th, 2022, a serial code card that lets players of Pokémon Sword or Pokémon Shield receive Genesect, Volcanion, and Marshadow will be distributed in the US, Canada, Australia and New Zealand. The codes were previously distributed in Europe in September 2022.

Receive Genesect, Volcanion, and Marshadow in Your Pokémon Sword or Pokémon Shield Game

Visit a participating retailer to claim codes that’ll let you receive these Mythical Pokémon.

There’s a trio of Mythical Pokémon waiting to join your travels through the Galar region—and to get them, all you need to do is visit a participating retailer in your region! There, you can get codes that’ll let you receive Genesect, Volcanion, and Marshadow in your Pokémon Sword or Pokémon Shield game.

Genesect and Volcanion

• US: GameStop (October 16–29, 2022)
• Canada: GameStop (October 16–29, 2022)
• Australia and New Zealand: JB Hi-Fi (October 21 – November 18, 2022)

Marshadow

• US: Target (October 30 – November 12, 2022)
• Canada: GameStop (October 30 – November 12, 2022)
• Australia and New Zealand: JB Hi-Fi (October 21 –November 18, 2022)

While you’re there, be sure to ask about preordering Pokémon Scarlet or Pokémon Violet, the newest titles in the Pokémon video game series and the first to feature an open-world adventure. Featuring a new region named Paldea, these games are filled with never-before-seen Pokémon and multiple storylines, and they introduce the mysterious Terastal phenomenon, which makes your Pokémon glisten like gemstones and can even change their types!

To claim your in-game gifts, follow these steps.

1. Launch your Pokémon Sword or Pokémon Shield
2. Select Mystery Gift on the X menu.
3. Select Get a Mystery Gift.
4. Select Get with Code/Password to connect to the internet.
5. Enter your codes one by one.
6. Watch as the gifts arrive in your game. (Pokémon will appear in your party or your Pokémon Boxes. Items will appear in your Bag.)
7. Be sure to save your game.

There’s no purchase necessary to receive these codes, but they’re only available while supplies last, so don’t delay! Please note that there is a limit of one code per Pokémon per person, and each code can only be used once.

Source: https://www.pokemon.com/us/pokemon-news/receive-genesect-volcanion-and-marshadow-in-your-pokemon-sword-or-pokemon-shield-game/