1. Login to your Outlook Web Access with your user name and password
2. On the upper right corner, click on your name. In this example, I will click on my name AJ:
Click on Open Another Mailbox button:
3. Type the name of the mailbox that you want to open. In my case, I will type the name of the shared mailbox that I have access to:
4. Click on Open. The new window or tab will have your mailbox open. If it gives any errors, it means, you do not have permissions to open that mailbox or that mailbox does not exist.