Setting Out of Office replies on Shared Mailboxes

To add an Out of Office (OOO) reply to a shared mailbox you must first have full access to the mailbox in question and then you can perform the following steps.

Log in to your Outlook account online at https://outlook.office.com/
Click on your photo at the top right hand corner of the screen
Select Open another mailbox
Type the full email address of the shared mailbox that you wish to manage and then click Open
Click Settings at the top of the page followed by View all Outlook settings
Select Automatic replies and create your OOO message