The best way to configure bonus articles, is in SecuOffice. You find the configuration at Loyalty » config cash register » Bonus products:
You need to configure one bonus product for every VAT rate. This is needed to calculate the remaining VAT properly. In the most cases you need one bonus product for the full VAT rate, another one for the reduced rate, and often one for articles without VAT. Stamps are exempt from VAT in Germany, for instance.
You can also define an article number or EAN for it, but these are optional. You need an article number or EAN if you want to inlude or exclude the bonus discount itself from from the bonus collection. You would then add it to the whitelist.