If you are trying to activate a copy you already own or are looking for a way to use the Pro features, here are the current options:
The Quest for a GameMaker 8.1 Pro License Key in 2026 If you are a retro dev or a hobbyist looking to fire up GameMaker 8.1 Pro
If you have a legal key from your childhood, back it up immediately. Treasure it. It is a piece of gaming history.