I forgot to add seats to my German/Austrian ticket
You can book additional seat reservations in our European train planner by selecting "Reservations".
Whether a reservation is mandatory depends on the type of train you take. Please see the overview of which trains require a mandatory reservation, and for which it is reccommended, on these pages:
Tickets in other countries either come with reservations as standard when bought via Happyrail, or have trains with no reservations necessary.