Open the Cost Estimate of the given project and click on the Edit discount button at the bottom.

Edit project discount
The following form will appear, allowing us to enter the discount.

Entering a discount for a project
Simply enter the discount amount and click Save.
How does the Discount affect Reporting?
The discount is always applied to the Price for people's work - it does not affect Project Expenses reports.
Specifically, we calculate the percentage discount from the project price and then reduce the of all items in the Price for people's work by exactly this percentage.