Featured image external url

Hi everyone!!

I’m having really big issues to set the featured image for the amp version of a post. I have my own plugin for gallery. I don’t use WordPress Gallery, so I don’t have a thumbnail imagen for each post. My images are in the same server, but another directory (inside plugin directory).

I’m trying to set @type”:”ImageObject”,”url”:first-image-of-the-post.jpg but I can’t.

I can get the url of the first image of the post, and I can show it automatically in my single.php file of the AMP theme. My problem is, I don’t know how to change application/ld+json to set that url.

I’m using: AMP 0.4.2 and Yoast SEO 3.1. which also can do changes on AMP version.

I hope you could understand me and help me. Sorry for my English.

Regards!! ??