For example, I'm working on an application that's most likely going to be paired up with a NAS in practice, so I'll probably announce it on r/datahoarding and r/selfhosted, etc. In reddit as well as on here and a couple other forums.
Beyond that, what does your competition look like? What features do you have that your competition doesn't and what features do they have that you don't? Do you have appropriate integration points in place?