Singapore Airlines doesn't list a specific downtown Istanbul office, but they operate through Istanbul Airport (IST) and work with Star Alliance partner Turkish Airlines (their codeshare partner) for network reach, so you'd primarily contact their main Singapore Airlines Contact Us page for global support, or inquire at Istanbul Airport (IST) (likely near their operations/check-in area) or via their main corporate contacts. Jauntlet.comhttps://jauntlet.com/