You will need to create a custom configuration file to accomplish this goal.
Here's how you do it:
Open Terminal (if it's not already open)
Create a new directory for some LightDM configurations (if it doesn't already exist):
sudo mkdir /etc/lightdm/lightdm.conf.d
Create a custom configuration file:
sudo vi /etc/lightdm/lightdm.conf.d/50-custom.conf