If you want to opt in to get ADF/XML data in addition to a regular lead data proceed with following steps.
1. Go to account settings at right upper corner.
2. Find option Notify me about every lead. If you have not been receiving leads before, choose this option.
3. At Additional Notification Data field select ADF/ XML.
As a result, as the new Lead is sent to email inbox, it comes with an attached data: