Step 1: Go to outlook.office.com
Step 2: Right-Click on the 'Folders' inbox
Step 3: Click 'Add shared folder'
Step 4: Type in the email address of the mailbox you want to add
Step 5: Click on the correct mailbox you want to add
Step 6: Click 'Add'
Step 7: You will now see your shared mailbox on the left hand side