There are many examples of using ducking discussed on the forum. In most cases there is some XML that has been posted.
I can provide you with a sample command, but I need to know how you are using VC. In particular, what are you using as a microphone setup?
Example: if you are using an Amulet Microphone, or VoxWav for Android, then you will have access to events which can be used to trigger the ducking.
If you are only using an "always on" microphone, then you probably need to either use a keyboard button, or a remote control to tell it when you want to speak, or you need to use a voice command like "pay attention", or a prefix. Depending on the scenario the ducking would be set up differently.
There are lots of options.