Hey @gabriele.limonta,
Thanks for your patience! For this to work you need to have Webinar Registration Required turned off, and then use the uel={base64EncodedEmail}
query param.
For example:
https://zoom.us/wc/123456789/join?prefer=1&un=bmFtZQ==&uel=ZW1haWxAZG9tYWluLmNvbQ==
Thanks,
Tommy