I run a marketplace buy-sell type SaaS app that is pretty niched down.
Cold outreach is key. We have a handful of competitors so really focused on "building a better mouse trap" then just reach out directly to sellers on competing platforms. Especially for the first couple dozen it's a hard sell, we had to basically give it away for free. Once there's an actual audience though it's easier to tell someone about it and just send them to the normal sign up flow with a discount code.
You can't get away from cold outreach though, especially at the beginning. I'm not sure you'd want to even if you could? The feedback for us has been invaluable.