First, before adding a camera to DVMS, stream encryption should be enabled in camera web UI:
Go to device settings
Go to the "Security" or "Safety" subgroup (name may differ for different devices).
In the settings, dialogue select the “System Service” tab
Enable the "Audio and Video Transmission Encryption" checkbox.
Then when you add a camera with supported and enabled encryption to DVMS, the driver automatically detects this stream and will add "AES-256 Encrypted" transport to System Manager -> camera settings -> Transport options. An encrypted stream will be used by default transport.