Step 1: Sign into your Office admin account via https://portal.office.com
Step 2: Navigate to Exchange admin center.
Step 3: On the left pane, scroll down and click More features.
Step 4: Click open under Permissions.
Step 5: Click Recipients on the left pane.
Step 6: Switch to the Shared tab.
Step 7: Double-click on a mailbox and switch to the email address tab.
Step 8: Click on the add(+) icon.
Step 9: Select email address type, enter the email address in respective box and click OK.