To buy your tickets with a promotional discount code, go to the ticketshop website and select the desired concert. If you haven't registered yet, you will have to do so in order to complete the purchase.
You will then have two ways to select your seat:
If you choose to select your seat using the map of the concert hall, you will see an option labeled Enter a promotional code on the right side. Click it an enter your code to activate the offer.
If you choose the option book the best seats, the program will automatically assign you the best available seats in your chosen zones, and you will still be able to enter your promotional code in the text box on the right side.