How can Apple Watch App Action be ran by automation?

I would like to allow users to run an Apple Watch App action by using automations on the iPhone.

The action can only be done on the watch and not the companion iPhone app?

I noticed that it's possible to start a workout on the default Apple watch app with automations, and also possible to start a workout on the 3rd party Dawn Patrol app using automations.

Is it possible to do this with custom intents or NSUserActivities or only the system start workout intent?