Outlook | Opening a Shared Mailbox in Office 365 Outlook Web Access (OWA)
To open a shared mailbox online, first login to the Office 365 Outlook Web Access using your normal staff account.
This KB Article provides the steps to open a shared mailbox in Outlook Web Access (OWA).
Click your avatar icon in the upper-right.
Click Open another mailbox...
Type the name of the shared mailbox, and select it. Click Open.