You can use the promo code by going to the in-app message that you received with promo code button. Click on the button within this message and you'll automatically go to the Buy credit screen. The extra percentage is then already added to the different top-up amounts and you can just choose the top-up amount that you would like to use.
You can also redeem your promo code in our Online Shop very easily. After you have entered your phone number in step 2 of your online purchase, you can enter the promo code in the appropriate field.