I have tried a few solutions to enable this option, but nothing is working. I am running Lubuntu 18.04 LTS
And what I have tried is
1)
sudo pcmanfm
/etc/xdg/lubuntu/lxdm/lxdm.conf
numlock=1
And
2)
sudo su -
apt-get install numlockx
echo "/usr/bin/numlockx on" >> /etc/xdg/lxsession/Lubuntu/autostart
And
Lastly 3) install numlockx via synaptic
sudo gedit /usr/share/lightdm/lightdm.conf.d/50-greeter-wrapper.conf
greeter-setup-script=/usr/bin/numlockx on
sudo reboot
However, after reboot the num lock key is still not on.

50-greeter-wrapper.confis the wrong file to edit on step 3 – Shayan Jun 22 '18 at 23:29/usr/share/lightdm/lightdm.conf.d/50-unity-greeter.confand then try to find this file on your own machine somehow. It will have a different name on your distro but the contents will be almost the same. – Shayan Jun 22 '18 at 23:32