There are two ways you can view a shared mailbox from the Office 365 web portal.
Option 1: Display the shared mailbox under your primary mailbox
Sign in to your Office 365 account at outlook.office.com.
Right-click the name of your primary mailbox (such as Betty Clark) in the left navigation pane and choose Add shared folder.
In the Add shared folder dialog box, type the name or email address of someone who has shared a mailbox with you, and click Add. Or, type the name of the shared mailbox you are a member of, such as firstname.lastname@example.org.
The shared mailbox displays in your Folder list in Outlook on the web.
You can expand or collapse the shared mailbox folders as you can with your primary mailbox. You can also remove the shared mailbox from your Folder list. To remove it, right-click the shared mailbox, and click Remove shared folder.
Option 2: Open the shared mailbox in a separate browser window
Sign in to your account at outlook.office.com.
In the navigation bar at the top right corner of the window, click your name. A list will appear.
Click Open another mailbox.
Type the email address of the other mailbox that you want to open and then click Open. Another Outlook on the web session opens in a separate window, allowing access to the other mailbox.
Note: If you mistype the email address of the mailbox, the result is "No match was found". Try retyping the email address.