JW Player for WordPress - Premium allows you to publish videos in two ways, adding the full url of the media content or selecting it from your videos hosted on jwplayer.com.
Videos hosted on jwplayer.com
This method requires the use of a cloud player and the API.
Please have a look here if you need more information:
Selecting the button Choose, you will have access to your contents coming from jwplayer.com.
You have just to click on the field Select video/ playlist... to see the latest single videos and playlists you've added on JW Player.
A click on the single item will make your choice, the thumbnail and the video information will be shown and you'll be ready to publish the content.
Self hosted videos
If you're using a cloud player, click on Add url for using this method, otherwise this will be the only available, even with the free version of JW Player for WordPress.
The full url of the media content is all what you need to publish your video but, if you need more, just click on Show otpions and see all the tools available.