There seems to be no option in the Options menu to allow the app to start automatically when the system starts. There is a simple work-around for this on macOS, but on Windows I can't find an equivalent.
This seems like an obvious feature to have, especially since the app already has a tray icon, and can start minimized to the tray icon. The immediate next logical step would be to have it start automatically.
Quick observation: the gist of this discussion should be added somewhere to the FAQ. That would be a good workaround for the absence of the autostart option in the app itself.
Secondly, I added the shortcut to Startup - and I ran into a bug. Joplin is indeed started automatically, but I get a big, completely blank window titled Joplin which is unusable and takes over the desktop. I close it, open Joplin again from the tray, and it works fine after that.
I have Joplin 1.6.8 on Windows 10.
The app is configured to show tray icon, and to start application minimized in the tray icon.
As for the shortcut in the Startup folder, I've tried two settings: launch the app with Normal window, and start minimized. Both trigger the bug.
You can probably get the window to show just by minimizing then maximizing (no need to close or exit). If we are discussing the same display bug, I was running into this bug with 1.6.x versions, but I don't think I've seen it since I went to 1.7.x (pre-release versions).