Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Doha to Medan as you normally would.
These are just IATA (International Air Transport Association) codes. They are used in aviation to identify and differentiate airports. That’s why a Doha Hamad Intl Airport to Medan Kuala Namu Intl Airport flight would often be abbreviated as a DOH to KNO flight.