"Shiny Eternatus will arrive in your game at Lv. 100 with a Timid Nature and the Pressure Ability. It’ll know the moves Dynamax Cannon, Eternabeam, Sludge Bomb, and Flamethrower, so it should make a powerful addition to your team!"
While the promo is technically suited for the UK region, and some users have noted that they are geo-blocked: others have been able to access it and utilize the email form. Note that you have until November 17 to take advantage (or until they run out), and you can tap someone you know in the UK if the site is giving you trouble.
How to get a free shiny Eternatus in Pokemon Sword and Shield