ScheduleOnce does not support third-party tracking widgets as is required for Wicked Reports optin tracking.
There are two possible solutions.
First, which is actually what we do and would be the recommended solution from our viewpoint, would be to have your prospects fill out an on-page optin form or survey - possibly in a similar popup using a modal form instead of an actual iFrame - for tracking and *then* pass them to ScheduleOnce to book the actual call.
Then, use this KB to pass the UTMs and contact email through to a custom landing page.
Finally, make sure your tracking widget is on that thank you page to grab the UTMs and email for tracking.