it's on the TDAC website QA. Where you start your journey, your departure airport. If you're departing from Manchester you put that. The flight number is the one get for plane you get off.
I'm putting UK, which is where I board for the start of my journey on one booking reference. I also printed my e-ticket. As the airline requested that. I decided putting the transit airport looked like I came from that country, so not going to do it that way.