This is becoming more common nowadays. Earlier it was a tiny “x” now you can’t even see the actual button.
I simply uninstall the app or the game whenever I get a full screen ad. Getting a full screen ad or an automatically sound enabled ad is not my thing.
Those keys are mostly ‘volume keys’ which are meant for devs. Some people sign up as devs and pay some joining fees and in return they get lots of Office keys and sell it on their website. You can’t use those product keys if you want to use Office on a different device as it’ll not bind to your MS account or can’t be activated on a different machine. You need to buy another key to use it on a different machine.