Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Kuala Lumpur to Dortmund as you normally would.
Dortmund (DTM), located 10.1 km from the city centre, is the airport you fly into when you book flights from Kuala Lumpur to Dortmund. 0 airlines have flights to Dortmund from Kuala Lumpur on a regular basis. Booking flights from Kuala Lumpur to Dortmund should not be difficult; the airport handles an average of 0 inbound flights from Kuala Lumpur per day.
This is defined as Kuala Lumpur Intl Airport (KUL) and Dortmund Airport (DTM). Codes like Dortmund Airport are often used to just call airports uniquely. Kuala Lumpur Subang Airport (SZB), Malacca Batu Berendum Airport (MKZ) are some additional IATA codes for airports related to your trip.