To add an additional mailbox in OWA you need to click on your name in the top right-hand corner. Click on ‘Open another mailbox’
NOTE: You must know the name of the mailbox you are attempting to add as well as have permissions to view it.
Enter the name of the mailbox in the ‘Name@ field and click on Open.
The mailbox will now appear in a new tab within your browser, separate to your own mailbox.
Some older browsers may show the shared mailbox in the left-hand side under your main inbox.