The YoYo Account is a one-stop login for all YoYo Games services, including GameMaker: Studio licencing (through Steam or the Desktop version), the GameMaker: Marketplace and the GameMaker: Player.
Older versions of GameMaker are not able to be used with the YoYo Account.
This now means Steam users can get access to the Standalone version of GameMaker: Studio.
You can find out more about the YYA (YoYo Account) here: Creating A YoYo Account
Note: Creating a YoYo Account is a manual process and you won't get one by buying GameMaker: Studio.
Tip: If you believe you have an account but are unsure then please try creating an account with your email address, it will then notify you if the email address is in use.