how do people send các câu hỏi to the show?

I don't think bạn actually send them directly to the actors.
In the ones I have heard of, the guy conducting the interview (be it directly cáo, fox hoặc any guy with a tv, radio show...) ask via his website, twitter and/or Facebook at his followers to send him các câu hỏi and he choose which one he will ask.
So bạn actually have to stay tuned and send your câu hỏi to the right person at the right time and hope for the best.