How do I add a shared mailbox on Outlook? (Desktop Version)
Firstly, request access using this form: Shared Mailbox Add / Remove
Once you have been given the relevant permissions to the mailbox;
Go to File then click on Account Settings and choose Account settings again.
Double click on your name/email address from the window that popped up.
Click on More Settings on the next window.
Lastly go to the Advanced Tab on the next window then click on the add button and type in the email address of the shared mailbox that you would like to add.
Click Apply then Ok, it should appear on your folders pane.
Linked articles
How do I add a shared mailbox on Outlook? (Online)
How do I reply or send an email from a shared mailbox address, rather than my own email address?