• Flash the Sonoff S20 with Tasmota.
• Configure the Module as 18 Generic
• Set GPIO0 as Button1
• Set GPIO3  as Switch1
• Set GPIO12 as  Relay1
• Set GPIO13 as Led1

Connect Door sensor to GPIO3(TX on Sonoff S20 board) and GND.

Since we want the power off when the door is open, we need to set the switchmode to inverted follow.

Via the console, set Switchmode to 2 (inverted follow 0 = on, 1 = off )

> Switchmode 2

All Done!