How could I find out childSensorId for the dummy sensor?

May I request value from Domoticz dummy sensor or user variable?
What childSensorId should be used in this case?

I understand Domoticz doesn't yet support MySensors smart sleep mode.
As a workaround MySensors node could periodically request certain value from Domoticz.
I cannot find any docs on how to define dummy device or user value in Domoticz those value could be retrieved by MySensor v.2 node.
My setup ...