See Microsoft document for opening shared mailboxes in the Outlook phone app: https://support.microsoft.com/en-au/office/add-a-shared-mailbox-to-outlook-mobile-f866242c-81b2-472e-8776-6c49c5473c9f
NOTE: The email account address of the shared mailbox is usually enough to open the mailbox, but if you are having issues, please submit a ticket and we'll verify your access as well as confirm the email address to enter into the app.