Pre-Build Pitch To Prospective Clients/Customers Template?

Hi,

I have an idea for a site I started working on. Before I go deep into building the app, research is telling me I need to reach out to prospective clients/customers to see if this is something they'd be interested in using/purchasing.

So I have people whom I'd like to reach out to in order to get their opinion and see if they'd be on board. It's worth mentioning, the service would be free to a list of people whom I plan on reaching out to. Problem is, I'm stuck writing this email, not exactly sure what to say. I'm afraid the difference between a yes/no answer could all be in my wording. Does anyone have a template they'd be willing to share for pitching your service to prospective clients/customers in order to gauge whether or not you should even build the app you're thinking of building?