The Free version of our player does not support HLS playback. The video embeds we provide in the account dashboard list your MP4 sources in order of lowest quality to highest. You should see those options by clicking on the HD button.
To set the default template for a specific player, I believe you use the template_key parameter on the /players/update endpoint but I have not tested this myself. Please see https://developer.jwplayer.com/jw-platform/reference/v1/methods/players/update.html for more details.