The value you type here is included after the @ in your initial %BRAND_OFFICE_365_SHORT% email address. If you type yourcompany ...

The value you type here is included after the @ in your initial %BRAND_OFFICE_365_SHORT% email address. If you type yourcompany, for example, your initial email address, which is also your user ID, will end with @yourcompany.onmicrosoft.com. After you type a domain name and click {0}, you'll prompted to type a name to appear before the @ in your user ID. 
If you have your own domain name, such as fourthcoffee.com, you can add it later, so that your %BRAND_OFFICE_365_SHORT% email address can end with @fourthcoffee.com. But for now, choose an initial email address so you can start using %BRAND_OFFICE_365_SHORT% right away.