GameMaker 8.1 was originally published by YoYo Games. The professional version (Pro) unlocked features like the ability to compile games into executable files ( .exe ) without limitations and removed the "Made with GameMaker" splash screen.
You can download the latest version from the official YoYo Games website. How to Safely Access Older Versions
YoYo Games officially discontinued GameMaker 8.1 Pro many years ago. Because the software is legacy, the original activation servers used to validate license keys are no longer active.