To add a shared mailbox to your Outlook Desktop App:
1. Open the Outlook Desktop App (icon here)
2. Once opened, click on File in the top left corner of the screen
3. On the next screen, there should be a box that says Account Settings. Click on that, and a drop down box should appear. Click on Account Settings in the dropdown box.
4. A new window should appear. Either double click on your email, or select your email and click on Change.
5. A new window should appear. Click More Settings on the bottom left corner of the window.
6. Another window should open. On this window, go to the Advanced tab. This is where you can add the mailbox. Click the Add button.
7. In the "Add Shared Mailbox" window that appears, type the mailbox that you will be adding to your account.
8. If done correctly, the email should show up in the mailboxes box. Click Apply in the bottom right corner. If Outlook shows "Not Responding" this is normal, and will resolve itself in around 10 seconds.
Once all of these steps are performed, the mailbox should be accessible in your Outlook Desktop App.