When you enroll in or upgrade to Apple Business Manager, you'll be asked to enter your organization’s website. Apple Business Manager uses the website you enter and will create a reserved domain as the default domain for your Managed Apple IDs. For example, if your website is www.mycompany.com, your Managed Apple IDs will look like email@example.com.
Your Apple Business Manager Administrator can change the default domain after upgrading as well as add additional domains. Any domains that are added after enrolling must be verified before they can be used for Managed Apple IDs.