The Application Certificate page of the Configure Office 365 Mailbox Access Wizard allows you to configure Email Signature Manager to use an existing registered app:
You will need to enter the Application and Directory identifiers for the app (which can be found in the Overview blade in the Azure portal), and specify the file for the Certificate to be used to authenticate the app (this can be the self-signed certificate generated by the wizard on the Application Certificate page). Alternatively, you can generate a new self-signed certificate, which will be saved to file and must then be uploaded to the app using the Azure portal.
When ready, click the Next button to apply the configuration and proceed to the Finished page, the Back button to return to the Using an Entra ID App page, or the Cancel button to close the wizard.