If you are currently not Premium, and you have purchased selected new Garmin devices (like the Edge 520/820, 735XT/935XT, and more) you are eligible to receive a Premium Trial.
First you must register the device in Garmin Connect. Then you should see an orange Strava tile on your Garmin Connect Dashboard. Click the link on this tile which will redirect you to the Trial sign up page.
If you are already Premium, or have troubles signing up for the Trial, please submit a support request.