You can't really do this with a Steam game; you'd need at least screenshots first.
If it's direct sales, you can do this with a powerpoint, as many have commented.
If it's via digital marketing, you can just test that they're clicking the ads. Or funnel them to a landing page where they click to buy or join a waitlist. Email marketing tends to convert very well to sales. If you can, you'd want to funnel everything into a squeeze page, where the only thing they can do is enter their email. Don't do it on a pop-up. You'll probably want to see where they're joining the waitlist from - is it under the hero banner, or the explanation, or the pricing page. This lets you know what they really care about.
This might be the old way of doing things though. More modern is to build a social media presence or get in front of someone else's and just propose something. If nobody is excited, then there's probably no market for it.