Here’s a quick way to add a shared mailbox in Outlook (desktop version):
  1. Open Outlook and click on the File tab at the top.
  2. Select Account Settings, then choose Account Settings again from the dropdown.
  3. In the Email tab, highlight your main email account and click Change.
  4. Click More Settings, and then go to the Advanced tab.
  5. Under Open these additional mailboxes, click the Add button.
  6. Enter the name or email address of the shared mailbox.
  7. Click OK to finish.

If by Web Outlook 
  • Log in to Outlook on the web.
  • Right-click the Folders section in the left-hand navigation pane.
  • Select Add shared folder (or Add shared mailbox, depending on your version).
  • Type the email address or display name of the shared mailbox, then select it from the results.
  • Click Add.