Underwear Girl 2, sequel to the original but now with more animation, dancing, music, and more.
This time, the "secret" isn't a code you input, but was a special version of the file you received from the author, who provided the file to us!
Here's a writeup by the author, who gives his blessing to his software being memorialized here!
Underwear_Girl_2.sit (1.04 MiB / 1.1 MB)
/ compressed w/ Stuffit
Emulating this? It should run fine under: Basilisk II