Originally Posted by
Bleepo
It seems to imply I can use it online but there are so many numbers on my voucher I wouldn't even know what to input.
It is a 13-digit number beginning by 057 for AF or 074 for KL.
If you have a FB account and are connected to your profile for a booking, your available EMD appear automatically in the voucher section of the payment page so you don't have to enter all the digits. At least this is true for AF website (not sure for KL).