I'm using WordPress REST API and can receive data from https://websitename.com/wp-json/wp/v2/posts?_fields=author,id,excerpt,title,link,featured_media_src_url.
The problem is featured_media_src_url is added with This WordPress Plugin and I can't retrieve it using URL filters. (it works by fetching just .../w/v2/posts without params)
It looks like the plugin gets the path using featured_media which returns an id. Adding this to the fields solves the problem:
https://websitename.com/wp-json/wp/v2/posts?_fields=author,id,excerpt,title,link,featured_media_src_url,featured_media