Follow the below steps to access "Shared Mailbox" in Outlook Online:
1. Go to your Outlook Online, and click on your account icon on the top right corner; Then click on "Open another mailbox" link:
2. Type in the email address of the Shared Mailbox yoy try to access in the box and click "Open":
3. If you have permission to access the Shared Mailbox, a new browser page will open for you to access the Shared Mailbox.