This event was distributed by serial code, to commemorate the release of the Virtual Console versions of Pokemon Gold and Silver for the 3DS.
The serial code was obtainable by those who purchased either game, and it was redeemed from their Nintendo e-Shop receipt.
After September 21, 2018, the code will no longer be given out.
For a limited time, a different card was distributed only to Pokemon Ultra Sun & Ultra Moon, which only allowed One Card Per Save.
That was fixed on Nov 20th,