I say you hold off on asking your manager for a reference until you know the company is interested in you and you are interested in them. I think it's important for you to really know as much as you can so you can decide if that company is a good fit for you. Ask about their organizational culture, do they provided professional development opportunities (improvement training, etc.) and the other things that are important to you. Since it's a new area it would be appropriate for you to ask about the area too.
When you do want to ask your manager for a reference I would do it in person. Explain to him why you are looking elsewhere and what your future plans are.
Best of luck tomorrow
I have my fingers crossed for you