Hello there Massimiliano,
I understand the importance of being able to obtain every data possible when it comes to your site however currently as you discovered it is not possible to embed a pixel to the Wix Bookings application.
As an alternative you can use Google Analytics Goals to Track Conversions in Wix Bookings .
We’re sorry for the inconvenience. We are trying to be as considerate as possible to our customer’s needs and requests.
Your feedback is greatly appreciated as we are always working to update and improve our products.
I have forwarded this feedback to the relevant department for further consideration.
This is also a known feature request, so I invite you to vote for this feature as well.
Wix Bookings Request: Adding a Conversion Pixel to the Thank You Page