Once you find out your screen identifier as Ben suggested using
xrandr then you can pick your desired display output (mine was called HDMI-1) and enter it direct in to the config file.
Search for the line "display-setup-script" and change it to read:
display-setup-script=xrandr --output HDMI-1 --primary
Obviously replacing "HDMI-1" with your desired monitor, eg. "DVI-1" or "DP-1" for DVI or DisplayPort if not HDMI.
I currently use lightdm-webkit2-greeter so I don't have this option, but if you use the lightdm-gtk-greeter then you can do the following instead:
In the file /etc/lightdm/lightdm-gtk-greeter.conf