How do I control whether Outlook performs native decryption of an SMime email?

Question

I am currently working on an add-in which takes over the encryption and decryption process of emails with the IPM.Note.SMime Message Class from Outlook.

To enable this, I am currently toggling a registry key located at:

Path: HKCU\Software\Policies\Microsoft\Office\16.0\outlook\security

Key Name: externalsmime

Key Value: 1 (to force Outlook to handle externally)

Are there any other methods by which I can force Outlook to handle SMime message files externally other than this registry key?

Please let me know if more information is required, thanks in advance for the help!

This is for Outlook LTSC on Windows.