The ESP module can read an analog signal and send its value to your controller.
The input signal needs to be connected to the ADC (also referred as Tout) pin on the ESP module. On the ESP-01, this pin is not available.
Connect a switch between ground and the ADC pin.
Use the device tab on the ESP Easy webinterface and create a new task by editing one of the available tasks. Select "Analog Input" from the dropdown box.
Enter the IDX found in the Domoticz device page. That should be all.