Add a "Shared" or "User" mailbox to Outlook in Office 365
You cannot do this yourself.
You will need permissions to access it. Please have the owner of the mailbox contact Computer Services by email to give their approval and also confirm if you should have "Send as..." access.
Once the request is completed you will see this mailbox will appear automatically on the left side of your Outlook.
Are you looking to make changes to out-of-office or inbox rules on a Shared Mailbox?