We offer automatic syncing. Visit PowerCenter and click 'Settings' in the top menu. Look at the side navigation and select 'CONNECT'. Click on 'Connect' under Garmin to connect your account. All of your workouts will automatically end up on the PowerCenter after you run!
The Stryd PowerCenter will only accept workouts with power data. Be sure to make sure your watch is setup so that it is properly collecting power data. You can learn how to setup your Garmin watch here.