If you add the permissions in Exchange, the shared mailbox should appear in the new Outlook under the shared with me folder, located in the sidebar, after a few minutes. You can expand it to see if this is the case. If it is still not appearing, you can try to add it with the following steps and check whether the shared mailbox displays in your Folder list under this folder.
Adding a shared mailbox to new Outlook
Open Outlook.
Hover your mouse over the Shared with me folder in the left navigation pane. A button with three dots will pop up. Clicking on this button will Hover your mouse overclick on the three dots and then choose Add shared folder or mailbox.
In the Add shared folder or mailbox. dialog box, type the name or email address of the shared mailbox, and then click Add.
You can view the shared mailbox under “shared with me“ option.
