You can integrate AdSense with your Legacy website builder website very easily. This feature is available in your Website Builder's Advanced section. Just hover your mouse over Advanced and drag-and-drop AdSense into your website. This field will have your AdSense adds.
Double-click the new field to edit its options. You can paste your AdSense HTML code there as well as change the field's size.
If you do not have an AdSense account and an HTML code generated, just head over to https://www.google.com/adsense/start to get started.