I recall, but can't find, a story on HN about someone who, before building any product at all, put up a website with an explanation of value, and a "purchase" p

I recall, but can't find, a story on HN about someone who, before building any product at all, put up a website with an explanation of value, and a "purchase" page. People put in their credit card number trying to buy the product, validating there really was some market for them.