Sites that make use of the Element API may not benefit from Craft's preview features if they're not making use of templating. This plugin adds a button to the Entry editing screen which links to a preview URL with
?version=$id parameters so your backend can look up content from the correct place.
It's your responsibility to write the backend code to fetch these revisions/drafts, but this plugin should allow you to give users a way to preview content directly from the editing screen.
To install this plugin, search for its package name on the Plugin Store and click “Install”.