Custom Field Mapping

Hi. Can you help me with the correct syntax for accessing and storing the enclosure URL shown on the right into the plugin as shown on the left? I have ben trying all weekend but must be missing something simple. I’ve searched the forum but can’t find what I am looking for. I’ve checked the documentation but I think I need more examples. Thank you.

https://drive.google.com/open?id=13xwq5On1cmpEvSz9u9osJ7inKuHR3XvA