From the job page Billing section, select Edit > Edit Billing Items.
Click into the field titled Add Items from Library or Add a Custom Item
Here, you can select a discount item from your Billing Library, or you can scroll down to Add Custom Item
Note: Click here to learn how to create a discount item in your Billing Library
To add a custom discount, select a type of discount from the drop-down menu
Simply name your discount, assign it a negative value, and Confirm
The discount will now show in the Billing section in green