The code is:

THENEOPETSTCG

(just copy&paste it)

The code is valid until Friday, April 26 at 5:00 PM NST and has to be redeemed at Grundo's Warehouse (https://www.neopets.com/space/warehouse/prizecodes.phtml)

The prize is a "Cool Negg" worth around seven million NP. Not "exclusive" like the AMA claims, but still pretty valuable.