Lined-up buyers.

Demonstrable proof that a market exists for this service; that people are willing to profitably pay for it.

How you do it, without having anything but 'vaporware', is you sell the initial round at a discount. These are your beta testers and in return for having the app customized to their preferences (because they will be the ones giving you feedback) and their testimonials, they get the lower price.

With orders in hand, you can then go look for funding.

Until that point, all you have is an idea. And ideas are a dime a dozen. People THINK their baby (idea) is valuable, but execution is what makes an idea valuable. By itself, without any market validity, it's pretty worthless.