when using a new custom prefix key (e.g. c_example) do I need to define this custom prefix key in AAM at all or will this data automatically be collected when using this custom prefix key value in a request?
@JulZur Since this is customer specific, ie: data sent in from your own properties, this will make its way into AAM by way of Server Side Forwarding (from Analytics to Audience Manager) but you still need to generate a trait that collects that data:
