2 methods:
During domain orders:
1. Search for domain names
2. Add the desired domain names to cart
3. Click View cart. You are redirected to the cart preview page
4. In the field Voucher code, insert your code
5. Click Add voucher code
In your EuroDNS account:
1. Go to My Account > Credit account
2. Insert the voucher code
3. Click Add voucher code