We understand your interest in uploading completed workouts directly to Garmin Connect. Unfortunately, Garmin does not currently allow 3rd party app workouts to be synced to their platform.
As a workaround, we have two steps you can follow:
1. You will receive an email from JOIN with your workout file, you can import this into Garmin Connect.
2. You can try exporting the Strava GPX activity file using the "sauce4strava" plugin. Once exported, you can manually upload the file to Garmin Connect. Instructions for this process can be found here. Please note that to receive badges on Garmin Connect, workouts should ideally be recorded with a Garmin device.
https://support.garmin.com/en-US/?faq=Ht3ZP52Kju075uKvqTqu99
We appreciate your patience as we work towards improving this functionality.