When adding a 'Shared Mailbox' as the Connection Account, it's not possible. You'll need to create a Service Account or Service Principal, ensure it has the correct license assigned, and any user logging into it to prevent multiplexing.
If you simply want to send an email as the Shared Mailbox, within the Outlook Connector for "Send an email," you can enable the advanced settings and use the "Send As" feature to send the email as the shared mailbox.