How do you promote a product while it's still in development?

1) Start building a community 2) Engage the community in your development process 3) Build a landing page 4) Write blogs posts regularly to start building your seo 5) On socials

You can always give a presentation of the concept, through mockups of the different features. Explain what it does and how much value it would bring to the table. Using Figma or adobe illustrator is also a good way to make a realistic demonstration of your app in development. It's quite cheap to make. I wouldn't call for promoting but more building a community around it. When you get the full version or MVP, then promoting seems plausible. Best of luck!