Hi Vincenzo,
Basically it boils down to access. Your BPM Server should have permission to access the Remote Server Destination. My guess would be that this HTTP destination should already be configured in your Server's NWA 'Destinations' settings.
The name which you define at the time of http destination setup, would be used inside the placeholders when you define the UI-URL while assigning the task.
So the name would be something like: "{name_of_http_destination_in_nwa_setup}/Relative-url"
Configuring the destinations would be somewhat similar to step#3 in this document.
Please let us know if you resolve this. Would be great to know something we haven't got a chance to try yet. 
Cheers Mate.