Altium 365 works on a named-user subscription. You authenticate with your email and password – no .alf file required.
Open Altium Designer and navigate to the page (available via the Current User icon at the top right). altium designer license file download
Go to the page (accessible via the user profile icon in the top right). Click Add standalone license file . Browse to and select the .alf file you downloaded. Alternative: Direct Software Activation Altium 365 works on a named-user subscription