I have an Oyster Annual Travelcard covering Zones 1 - 4 with Network Gold Card. I've tried the sites for Chiltern Railways, Southeastern Railways, National Rail and Trainline (I suspect they all use the same booking database?) but I can't see how to book a ticket online from Boundary Zone 4 to High Wycombe (I'll be travelling from Marylebone). They do allow you to apply the Network Gold Card discount, but don't seem to allow for the "MYB-BZ4 credit". Presume I'll need to buy the ticket at the booking office at MYB?