Still, if you feel you must send them a sample of your written English, you ring the company, and say, "Could you help me, please? I have an interview with X next week, and I want to send a thank you note. However, I don't know the address to which I shoud send it. Would you be able to tell me, please?"
Or, if you need the address for the interview, as your topic line suggests, substitute "and I don't know the address at which the interview is to be held."
Student or Learner