Found on Reddit. Giveaway via emailed codes from Game UK website and twitter (but works internationally as codes are not region-locked):
We are pleased to offer Shiny Zamazenta in Pokémon Sword and Shiny Zacian in Pokémon Shield from 22nd October – 18th November 2021.
All you have to do is enter your email address below and we will email you your unique codes. Your codes will allow you to receive Shiny Zamazenta in Pokémon Sword and Shiny Zacian in Pokémon Shield.
No purchase is necessary
(Please note: We will aim to email your codes every Monday. Only 1 code per customer will be sent)
USE THESE STEPS, ALONG WITH YOUR CODE, TO RECEIVE YOUR SHINY ZAMAZENTA IN POKÉMON SWORD AND SHINY ZACIAN IN POKÉMON SHIELD.
1. Launch your Pokémon Sword or Pokémon Shield game.
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 code.
6. Watch as the gift arrives in your game. (Pokémon will appear in your party or your Pokémon Boxes. Items will appear in your Bag or in the fitting room of any boutique in the Galar region.)
7. Be sure to save your game.