Okay! l did some digging around, and this is what l could find, to be the earliest Pokemon event!
lt was the "Legendary Pokémon Offer Mew" given away to Pokemon Red and Green versions, in Japan!
l don't know if that's the first (it seems like it!), but that is the earliest event l could find!
This promotion received over 78,000 applications, but only gave away 20 Mew.
The level of the Mew was unknown, and it came with a Bitter Berry, knowing only the move Pound.
(+) This event dated back to April 1998 during the distribution time.