The Action Button on Apple Watch can be customized to starting a workout or creating a compass waypoint.
Open the Apple Watch app on your iPhone, tap Action Button.
Now tap the Action menu and choose a new action for the button.
Here's a look at the choices.
You can start a workout,
use the stopwatch,
create a new Compass Waypoint,
or use Backtrack to retrace your steps if you get lost.
You can also start a dive session
or turn on the flashlight.
And that's how to customize the Action button.