Start By Making The File:

nano shutdown.py

And Paste In The Following Code And Save:

# !/bin/python
# Reboot/Shutting Down A Raspberry Pi Using A Momentary Switch

import RPi.GPIO as GPIO
import time
import os

GPIO.setmode(GPIO.BCM)
GPIO.setup(4, GPIO.IN, pull_up_down=GPIO.PUD_UP)   #Change GPIO PIN If Needed
def Shutdown(channel):
    print("Reboot/Shutting Down")
    time.sleep(3)     #Time In Seconds Before Reboot/Shutdown
    os.system("sudo shutdown")   #reboot/shutdown command

GPIO.add_event_detect(4, GPIO.FALLING, callback=Shutdown, bouncetime=2000)   #Change GPIO PIN If Needed

while 1:

    time.sleep(1)

sudo nano /etc/rc.local

And Add The Following Line ABOVE THE EXIT 0 LINE And Save:

sudo python /home/pi/shutdown.py &