The additional 20% PRO discount is applied directly to the courses, you don't have to do anything! As it is additional, you can use another promotion or discount code in your order.
When you log in with your PRO account, you will see all the prices with the 20% discount already applied but once you add the course(s) to your shopping cart, you will see the price broken down in addition to any other promotional codes added.
Please note that the PRO discount is not added to other discount percentages, but rather the PRO discount is applied to the current course price. For example, if the course has a 10% discount (due to a special offer or a discount code has already been applied), the additional 20% would be applied on top of the already discounted price.
Remember that the PRO discount is only valid for purchases made on the web and it applies to individual courses that you buy to realize from your own personal profile, it will not be applied on Course Bundles, Gifts or any purchase made through the Domestika app.