Adding srcset to an image in ajax

wp_get_attachment_image_srcset is the key here:

wp_get_attachment_image_srcset( int $attachment_id, array|string $size="medium", array $image_meta = null )

I believe the missing information you lack, is that post thumbnails are actually an attachment ID. In combination with get_post_thumbnail_id you should be able to generate srcset values.

But also keep in mind that while googling, every question I found was asking how to remove srcset not add. Apparently what you wanted was actually added in WP 4.4. The answer might actually be to not set srcset at all, and let WP set it for you.

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