If you have been granted access to a shared mailbox, close and restart Outlook. The shared mailbox should automatically appear in the Folder Pane.
Please note that if your administrator has only recently assigned you access to the shared mailbox, it may take several minutes for the permissions to synchronize. If the mailbox does not appear immediately, wait a few minutes and then restart Outlook again.
If the shared mailbox still does not appear, you may need to add it manually.
How to Find the Shared Mailbox in the Folder Pane
In the Folder Pane on the left, look for a folder named Shared with me or a separate mailbox listed below your primary email address. Expand the mailbox to display its folders, such as Inbox, Drafts, and Sent Items.
How to Add a Shared Mailbox Manually
The shared mailbox should now appear in your Folder Pane.
Note: Before you can access a shared mailbox, a Microsoft 365 administrator must grant you permission to the mailbox.