Sitemaps and SEO

Thank you for an easy route into generating AMP pages. I have one issue:

Is there a particular recommended way to get the AMP pages added to sitemaps?

I use the Yoast plugin which generates a group of Google-friendly sitemaps under sitemap_index.xml but I don’t see it picking up on the AMP pages.

Any ideas on this?

https://www.ads-software.com/plugins/custom-amp-accelerated-mobile-pages/