This event was distributed to international players via a 3DS region-locked QR code on the main Pokemon website.
For Japanese players, it was distributed via a QR code on a Ga-ore disc with the QR code on it, collected only in theaters for the Volcanion and the Mechanical Marvel Movie. (QR Code can be found in the screenshots segment of this posting)
This event is actually pre-programmed into the Pokemon Sun and Moon games, and it requires only the QR code to activate,
after the player has cleared the game (become the Champion).