If you have multiple accounts in the Outlook app, under Select an account to log in with tap your primary work account; the one which has permission to access the shared mailbox.
Enter the email address of the shared mailbox you wish to add, then tap Continue.
The shared mailbox will display in your account list in the Outlook app.
To remove a shared mailbox, go to Settings > Accounts. Then tap on the shared mailbox and select Delete Account.