After you set up a custom domain in Office 365 (like fourthcoffee.com, for example), you can change the default domain used for new email addresses when you add users in Office 365. You must have at least one custom domain that you've added to Office 365 before you can choose a default domain.
To choose a new default domain for email addresses:
Sign in to Office 365 with your work or school account.
Choose Setup > Domains.
On the Manage domains page, choose the domain you want to set as the default for new email addresses.
On the right, click Set as default.