Rated 4 out of 5 stars

Hi, there,
I completely agree with Karel Klapka: this add-on works, but of course there's room for improvement (as the author says).
Negative comments came mainly from the fact that the action button wasn't in the right place - it should be on the calendar page.
There may be a solution for that:
Add line: "allowed_spaces": ["calendar"] under: "browser_action" in manifest.json