It´s possible to define "Payment discount" in two ways:
- Date
- Number of days
Adding the percentage under "Discount".
Note that different recipients may require different ways of defining the payment discount.
Date
To start setting a "Date", click "Select date".
A calendar will be shown. 
It´s possible to select year and month... 
...and day to select the date. 
The date will be selected. 
Number of days
Click the field for "Number of days" to start adding number of days.
Add the number of days, manually or by using the up and down arrows. 
The number of days will be added. 
Discount
Click the field for "Discount" to start adding the discount percentage.
Add the discount percentage, manually or by using the up and down arrows. 
The discount will be added. 