Quick answer: however you like! If you have any feedback, suggestions or insights, or if you want to report a bug - create a topic describing it and you’re done!
HOWEVER, there are few things that might be easier for both you and us:
Use a proper tag
You can go for focus-suggestions for feature requests, insights and any sort of feedback.
Use focus-bugs to report any issues with Toggl Focus.
Check if a similar feature request/bug already exists
Not required, but it’s easier for everyone to track things that really matter, because…
Vote for things you agree with/have similar issues with
…for each topic you can leave a 
- that way we can see if a certain feature is really wanted by many of you, or if some bug is an issue to a wider audience.
And that’s it! Happy community-ing! 