After you entered your card number and personal details the voucher code will be showed on the next page. The code is just valid for bookings via FlixBus App.

ISIC serial number
E-mail address
Your e-mail address is used only for delivery of your voucher and will not be used for any other purpose unless permission is given.
Your name (as stated on your card)
  I agree for ISIC Global Office to use my e-mail address for the purpose of market research.