After you've created your design and the preview looks good, you can create an enriched feed URL to copy over into your commerce manager.
From the ad builder preview page, click on Publish Feed on the right. You'll be prompted to select an existing output feed or create a new one.
When you make a selection, you will then be redirected to the output feed that you've added your design into. From here you can click publish in order to render the output with the added design or you can choose to add more designs before publishing the changes to the output feed.
Once the feed is generated, click Copy Link and add it into your Commerce Manager to begin using your designs!