If you’re able to access your chat logs, you should be able to take a screenshot from there :)
Assuming you’re on a Windows device; press the windows key + R, type in %appdata%, click on your .minecraft folder, then logs, then the latest session (if that is when you won the key).
You may need to close down your game if it’s the current session, and re-load it for the logs to register. If you’re not on Windows, let me know and I can give you another solution to get to your logs.