Importing media files from custom CMS into PowerPress

Hi, I’m importing some posts from a custom CMS, I’ve generated the XML and I’m using WP All Import to do the migration. In my custom CMS we have the field with the media file from our CDN, how can I import it to PowerPress? It’s stored in a custom field? If yes, what’s their key?

I’ve tried to figure out looking at source code, but was unable to get it.

Thanks!