Outlook on Web - Add Shared Mailbox

1) Browse to outlook.office.com/mail

2) Right click on the "Folders" option above 'Inbox'

3) Select "Add shared folder or mailbox"
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4) Type in the Shared Mailbox name and select it from the dropdown.

5) Select "Add"

6) The shared mailbox should now appear at the bottom of the pane on the left side