Using WordPress’s default custom fields in AMP

Hello, i’m trying to add a amp-iframe element after post title in posts. It changes every post, so i need to use custom fields. I saw a topic in this forum (https://ww.wp.xz.cn/support/topic/using-wordpress-custom-fields-in-amp/) but there is not enough information.

I want to add this:

<amp-iframe width="200" height="100"
    sandbox="allow-scripts allow-same-origin"
    layout="responsive"
    frameborder="0"
    src="HERE COMES LINK OR TEXT ELEMENT VIA CUSTOM FIELDS">
  </amp-iframe>