The widget support all four google analytics UTM parameters:
- utm_source
- utm_medium
- utm_content
- utm_campaign
In order to pass them, just add them as get parameters to the widget loader:
<script src="http://whitelabeldemo.skyscanner.net/scripts/widget/sswidget.load.v1.min.js?&product=flights&utm_source=some_parameter"></script>