that's not on the TDAC question and answer info. They say the country you embark on your journey i.e the country you are coming from. I'm not putting that my journey starts in Delhi when I've not been in the country. My flight originated in UK, so that's my departure point.
It's on the website. You do it online up to 3 days before landing. Wherever you are. Even in the airport on arrival before immigration, but that'll delay your entry.
Someone already posted that the Q&A on the TDAC says best to put departure country airport (like UK) not the transit one, but the flight number is the one for the plane you get off.
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.