Building a pre-launch email list without running ads

What's worked or hasn't worked for you folks when building a pre-launch email list? Are ads necessary?

For context, I'm launching a meal prep company to help individuals with chronic illnesses eat healthier and reach their health goals. Since my target audience is somewhat niche, how can I reach them without running ads?

The goal of my pre-launch email campaign is to gain interest in my product and to also do market research to find what my potential customers would like to see before the official launch(i.e. features they'd like to see, products they'd like to see, etc.). Thanks!