This settings is used to determine whether a user is an Office 365 user or not, assuming that the user’s login name is equal to the User Principal Name which is equal to the user’s email address.
To see your custom domains you can navigate to the Azure Portal > Azure Active Directory > Custom domain names.
To support enterprises that have mapped multiple domains to their Office 365 tenant, you can add more than one custom domain.
Only fill out if you added a custom domain e.g. mycompany.com to your Office 365 account, otherwise leave empty. Do not add the cloud-only yourcompany.onmicrosoft.com domain here.
Important After you entered a domain name in the input box you must click “+” to add it. Also when you are entering just one domain.