Since Outlook for Office 365 came out Microsoft have removed the ability to add accounts where the username is different from the email address. This means that you have to add the account via the Windows Control panel rather than through Outlook.
Click [Start] and type “Control”
Click [Control Panel]
Type “mail” in the top right corner, then click “Mail (Microsoft Outlook 2016)”
Click “Email Accounts”
Click [New] to add a new account.
Click the “Manual setup” radio button then click [Next]
Click “POP or IMAP” and then click [Next]
Fill in your account details then click [Next] to add your account.