What is a good place for launching a prototype for validating an idea?

Based on what I've seen and tried, I'd recommend switching mindset from validating ideas to solving people's problems.

Whose life are you going to make more effective?

Who will save money because of your product?

Who will get things done faster?

If answer is "pretty much everybody" then use places you already know – where people willing to try new products spend their time. PH, Reddit, DN, HN, IH, you name them.

But I'd recommend going niche. Find a small group of people that you want to target and go to the places where they hang out.

Forums, Telegram groups, niche websites, Instagram profiles, etc.

50 people with problem that you're trying to solve are always a better audience than 1000 who never or hardly ever experienced the problem you're solving.

Especially on early stage.

And remember – your prototype in most cases better solve ONE problem really well.

Don't try to build many features at once just to see which one sticks.

Go one by one.

Test solutions to people's problems and don't be afraid to launch product in front of the same audience multiple times.

Hopefully, they will be grateful later.

Good luck.

wow, it is even better what I expected to hear, thank you so much! :)