Hi Uli,
My name is Shweta and I am from audience manager support team.
I would like to clarify here that AAM’s URL Destination is essentially a partner’s(the platform receiving the data e.g. Google/Facebook or other targeting platform that the client wishes to send segment data to) own pixel that is fired by AAM on behalf of the partner, used to transfer segment data to partners.
URL destinations are typically used for offsite targeting and require a site visit in order to pass the data to the partner.You should not use it for passing CustomerIds. For that matter,you can utilize our DCS API calls:
DCS Event Call APIs
Thanks,
Shweta