Making a snap auto-start

What is the best approach to make a snap auto-start after login?

See also:
How to add 'autostart' to the app (no answer)
How to autostart a snap of a desktop application? (not sure there’s a solution here)

The snap format describes the autostart: property of the application declared in snap.yaml (or snapcraft.yaml for that matter). I think it should work just fine with snapcraft 3.0, but in case you get into trouble, try placing autostart under passthrough.