How to brainstorm an idea on Indie Hackers?

Well usually it's best not to share an idea, but rather something to see/work with.

and show it.

Showing is better than Telling.

Make something
Show something
Tell us.

Well usually people would just share their idea and then see what the community here thinks of it...

This could be a good starting point to self-brainstorm Micro SaaS Ideas

But to answer your question - The best way would be to spin up a landing page and post it here on IH as 'Show IH'. If you have more bandwidth, you can build Figma Wireframes or a small prototype to brainstorm further with audience here on IH.