Post about it where your user-base (people that care about their privacy) may be.

Places like:
https://www.reddit.com/r/privacy/
https://www.reddit.com/r/privacytoolsIO/
The forum' Service Announcement board (people also appreciate their privacy here).

If it's a product, you may also see if it's a good idea to create a signature campaign here (depends a lot).
If it's also related to Bitcoin, post on /r/Bitcoin or /r/CryptoCurrencies (if related to any other crypto, or all of them in general).

Just some ideas I could think off.