https://smb.sega.com/bananamania/lang/en.html#section__incentive
There can be some regional Issues:
Subscribe to the newsletter and choose which platform you would like to receive the key for. There is a mention about "only available in selected regions", there is a country selection menu for the newsletter (though you can select anything).
Steam package you receive - https://steamdb.info/app/1647320/
Base game: https://store.steampowered.com/app/1316910/Super_Monkey_Ball_Banana_Mania/