Help radicale calendar with Zimbra external calendar integation

Hello,
I would like to know if it is possible to integrat a caldav calendar (hosted on a radicale server) with zimbra (open source) as external calendar ?
I tried many time to integrate it in zimbra without success.
Zimbra ask for a email adress, a password, and then the url to the external caldav calendar.

May you help me ?
Best Regards,
EdG