Go to the app store, email developer, write it in the review.
If it's on GitHub/BitBucket/... you can request features there. Else, mail the developers or contact them if/when they host a Reddit AMA.
Shout it into the void. Unless it's groundbreaking, odds are its already been discussed or on a road map
This. Odds are the person getting paid to think about it 40+ hours a week is going to cover the idea you thought up in the shower.
You can do it here. Keep in mind, we have a huge list of speculative ideas that we donβt have the time or resources to do. We also have a dumpster full of ideas that we tried, measured and tossed away after they did not work well enough. So there is a fair chance your idea will already be in one of the two piles.