[alsadi]

By reaching them via official website/email. You are aware that most software have official website, have readme with some copyright note, some authors file...etc

Yes, it's a policy Quote

If there’s an app that you'd like to be distributed on Flathub, the best first course of action is to approach the app’s developers and ask them to submit it.

https://github.com/flathub/flathub/wiki/App-Submission