How to access shared mailbox in OWA. Description: This page contains information and instructions on how you can access a shared mailbox in OWA. Solution: Open a web browser and access the Outlook Web App. URL https://outlook.office365.comSign in to the portal using your email address and password.On the upper left corner of the screen, click on your user name and click on “Open another mailbox” from the sub menu.Type in the mailbox that you would like to access then select the mailbox from the list and click open.