Some time it is hard to find solution for raspberry pi.
This time we are solving issue : how to Power Down Your Raspberry Pi using GPIO.
copy script from power_hold.py
sudo apt install python3-gpiozero #connect button GPIO 21 and GND #run below command to test python3 power_hold.py # hold button(gpio21 and GND) for 2 second . your pi will be shutdown.
If it is ok then power on your pi3 again
Now execute your script ,when raspberry pi power on every time.
sudo nano /etc/rc.local
in rc.local file at the end your code must be look like this
python3 /home/pi/power_hold.py &
Thats it now . restart your pi again and test it….