This should work, add somewhere between other functions in /wp-content/plugins/wp-custom-html-pages/wp-custom-html-pages.php
function WPCHTMLP_post_type_link($post_url, $post) {
if ( 'wpchtmlp_page' === get_post_type( $post->ID ) ) {
$page_meta_box = get_post_meta( $post->ID, 'WPCHTMLP_page_meta_box', true );
if(isset($page_meta_box ['html_permalink']))
return get_site_url()."/".$page_meta_box ['html_permalink'];
}
return $post_url;
}
add_filter( 'post_type_link', 'WPCHTMLP_post_type_link', 10, 2 );
It will add filter to permalinks, where if post type is custom HTML page and it has custom permalink setup it will change default permalink to website.com/custom-permalink.