Help setup Master device in scenes

[jackwan1]

Have been using Domoticz for years, first attempt to setup a scene (not groups)

I have a dummy switch that is controlled by http commands from another app. It is working fine.

how can I use this dummy switch as a Master to turn on/off some sonoff switches that is currently working in Domoticz as "Slaves"?

Below are all from UI when setting up a "scene" in Domoticz:
In the "activation device" section, to add a device, I can only "push a bottom" on the remote. Domoticz does not allow me to manually add a device, which suppose that will be a Master and the slaves are to be entered in the "Device" section. Based on my research, that is how to setup a scene, please correct me if I am wrong.