Ask HN: I have a list of 20 potential customers. How to reach out to them?

My idea is very niche focussed and related to my current field. I know quite a few people in the field already. So I have this list of about 20 people that I want to start a discussion with. How would you go about it ? Should I just use something like survey monkey ? personalized email ? custom form ? phone call and ask questions ?