What was the first video game Easter Egg message on an Atari game? Bonus points if you know why it was placed there!

The Adventure game secret room is believed to be the very first software Easter egg. Warren Robinett created it to hide his name because of Atari's policy of not acknowledging its game developers.

https://atari.com/blogs/atari/so-let-s-talk-about-that-easter-egg

"Adventure" (released in 1980) was the first ATARI (!) game to contain an easter egg. However, this was not the "very first software easter egg". There had been earlier easter eggs in games on different platforms. The earliest known video game easter egg appeared in "Moonlander" (1973), a game in which players try to land a module on the moon. If a player flies their ship horizontally in the close-up view, they encounter a McDonald's restaurant which the astronaut will visit upon landing and can destroy by crashing into it.

Other early video games containing easter eggs are "Colossal Cave Adventure" (1976) or "Starship 1" (1977). The first easter egg in software in general is even older. It dates back to 1967 and was hidden in a text editor called TECO. In this editor, in order to create a new file you had to use the command "MAKE" followed by a single-word name you had chosen for your file. However, if you intented to create a file named "love" by typing "MAKE LOVE" the computer would surprise you by responding "NOT WAR?" before you could proceed.

Sources:
https://www.acriticalhit.com/the-true-first-easter-egg-ready-player-one-was-wrong/
https://www.acriticalhit.com/make-love-not-war-first-software-easter-egg