This gift is distributed to players via a serial code, given in participating stores in Europe, from 25 September to 25 October 2020.
The codes are redeemable until January 31st, 2021 (Thanks @AlamosIT for the report).
Using the Dynamax Crystal would allow the Pokémon to appear at the Watchtower Lair's Den as a max raid.
The Pokémon can be Shiny with standard odds. Attached there are the wonder card, the standard Pokémon and the Shiny variation.
To be able to use the items associated with the event, including injecting items into the save and getting it to work,
the player's Switch must be connected to the internet, with the Mystery Gift>Get Wild Area News option used during the event period.
Alternatively the player must have connected to the internet and redeemed a Mystery Gift during the event period.
This gift can be redeemed on either Sword or Shield.
This events also has a certain fixed IVs.
As of time of writing, end date for the serial code redemption is presently unknown.
In order for the item (whether redeemed or hacked in) to function, players would need to have at least Wild Area Event #24
downloaded into the save.
(When collecting a Mystery Gift, a copy of the Wild Area News would be downloaded to the save)
As of LGPE, wonder cards are no longer stored in saves. As such, players will not be able to inject this event into their save using PKHeX.
Original news with more details: