After searching through the wordpress docs, I found what I needed. What I was missing was the hook called “the_content”. My function, and all the code I wrote inside it, was correct. I just needed this stupid hook.
function displayIframe($content) {
//[my code to get the $currentPostID and $currentPostGuid]
//...
//...
if (is_single( $currentPostID )) {// if on the specific custom post page
$content .= "<iframe src="" . esc_html($currentPostGuid) . "" width=100% height=900></iframe>";
}
return $content;
}
function main() {
// the_content hook was what I needed; I had no idea it existed
add_filter("the_content", __NAMESPACE__ . "\displayIframe");
}
I hope this is helpful for others. Goodluck 🙂