First
Copy the backligh.sh to /home/user/
Second
Make the backlight.sh executabble by opening the terminal and typing this:
chmod +x backlight.sh
Third
Open and edit the backlight.desktop, on line 4 'Exec=/home/mrepol742/backlight.sh' replace 'mrepol742' with your username.
Forth
Copy the backlight.desktop to /etc/xdg/autostart/
DONE
it wont gonna work again if the keyboard is disconnected consider executing the backlight.sh or xmodmap -e 'add mod3 = Scroll_Lock' in terminal.
i will update this if i found a way to execute the backlight.sh whenever a new device attach
im also a linux user whos also facing this kind of issue