Press 1P Left + 1P Right + 1P Fire + 2P Start, then 1P Left + 1P Fire + 1P Start during the game demo to make the text "BANDAI and TAITO COP" [sic] appear below the high score.

https://tamagotchi.fandom.com/wiki/My_Tamagotchi_Forever

My Tamagotchi ForeverTamagotchi Wiki