If you already have the option switched to Outside for that particular workout then it should automatically push to Garmin. It may only do it 2-3 days in advance though so that may be why you don’t see it yet.
Make sure that you have done everything in the articles below to push to Garmin and you should be set.
Make sure that you’ve everything set up with the right permissions (the Authorise Garmin Sync steps). Click the Outside button for your workout. Wait a few minutes for the various services to sync up. Turn on your Garmin and after a minute or two you should see a message like “Download completed”. It doesn’t get put in the Workouts sub folder (that’s for Garmin’s own workouts) but the “Training Calendar” folder. It won’t be downloaded if it’s too far in the future.