Before you proceed, it’s important to make sure you have the necessary permissions to access the shared mailbox or the other user’s mailbox. Now that you’ve confirmed your permissions, follow these steps to easily add the mailbox to your Outlook Web App (OWA):
Sign-In to the Outlook Web App (OWA) by opening a web browser and going to https://outlook.office.com/mail/ and using your Microsoft 365 credentials.
Locate the Folders Pane and right click on it. A menu will be displayed. Select Add shared folder or mailbox.
A pop-up window will appear. Type in the email address or name of the shared mailbox (or user mailbox) you want to add and select it.
Click Add to complete the process.
The shared mailbox will now be displayed in your OWA folder list. To locate it, scroll down the folder list in the left navigation and > Mailbox Name to expand the added mailbox and see its folders.