I don't have Zorin OS 8 to test but to do this in version 9 do the following:
Open Terminal and type the following:
sudo nano /usr/share/lightdm/lightdm.conf.d/50-zorin-greeter.conf
Then add in at the bottom line:
greeter-show-manual-login=true
Save and reboot, you will now have a login option which will ask for username and password.