I also have the same problem. Based on this link https://developers.facebook.com/docs/marketing-api/reference/ads-action-stats/, meta provided some parameters for developers to pull the appointments scheduled data. I tried to use schedule_total and schedule_website since the ads campaign is based on external website/landing page, and none of them works. It's been a year now, so perhaps you found the answer. I will be very grateful if you are willing to share it with the rest of us