How to engage target audience on social media before a product launch?

The discussion part of the product hunt is a nice place to start. You can also look at upvoter of a similar product to yours, and connect with them over Twitter.

Also looking into this for the project I'm currently working on. I'm trying to provide as much value as possible through different types of content and engage with people who I think would be interested in my service. Currently, I'm active on Twitter the most.

I can share what I am doing. I have created an Instagram account (@chemistry_experiments_videos , @chess_master_tutorial )for each mobile app that I have made. And I upload short videos every week with related content in order to find targeted users. Now my accounts are reaching 10k followers each and every video has tens of thousands views!

You can build an audience among your target customers, but it's important to provide them with a lot of value before trying to convert into sales. Some people also "build in public" where they document what they are working on, sharing the successes but also the failures, as they learn on the way.