Somewhere in your phone are the people who already said they would use this. Past clients, friends with the right problem, the person at a party who said it sounded like a good idea.
Message them individually — not a broadcast — before you open, with a date and a price. Ten paying customers in the first week changes everything: it pays for the launch, it produces your first reviews and photographs, and it means you are refining something real instead of waiting by a silent phone.