How, where, and when do I make reservation for High speed trains?

Lennart Petersen schrieb:

Seem to be correct. I don't know about any railway at all allowing
reservations only on-line.

German Rail allows on-line booking of reservations without a ticket.
Choose "No ticket - Reservation only" when booking; major credit
cards are accepted for the reservation fee. You have to register
to book online, however.

Long-distance ticket machines also allow to make reservations without
buying a ticket.

.... Martin