To check if there is a special promotion for the network provider you want to recharge, all you need to do is select the country code.
The current top-up promotions will appear automatically on the screen:
You can click on the promotion to read all its terms and conditions; for example: some promotions require a minimum amount to be applied (say, $7). If your recharge is valid to qualify for the promotion, the promotion will be applied automatically when you finish your top up and you won't have to do anything else, unless otherwise stated in the terms and conditions.
Also, you can obtain additional discounts on your top-ups by using your doctorSIM credits: