There's this PR I was working on that adds support for URL links.
It's done for Linux and Windows but for macOS it's not working. I have no experience with macOS and don't even have dev environment set up, so can't make any progress here.
open-url is called when the app has already been started. However, when the app is not running, opening the joplin:// URI will start the app, but open-url is not called.
Thus, when Joplin is open, everything works perfectly. When Joplin is not running, a joplin URI will only start the app, but will not do anything else.