In the space dungeon, collect space treasures and avoid space monsters, then run to the space goal. Space.
To trigger these credits, hold Up + Left on the left joystick, Down + Right on the right joystick, and 1P Start + 2P Start together. If done correctly, a dot should appear near the bottom of the screen.
The following text is at 0xFAF2 in the main CPU. Unlike the hidden credits, there doesn't seem to be any trigger for this.
STOLEN FROM THE GOOD BUDDY'S AT TAITO BY AN UNAUTHORIZED DO BADDER