Custom fields for post title on single.php

I want to use a custom field in order to put HTML tags into post titles and have found suggestions that say I should use the code below on the single.php in place of <h1><php the_title(); ?></h1>:

<?php
   $html_title = get_post_meta($post->ID, "HTML_title", true);
   if ($html_title) { ?>
      <h1><?php echo $html_title; ?></h1>
   <?php } else { ?>
      <h1><?php the_title(); ?></h1>
<?php } ?>

The issues is that my current single.php doesn’t have `<h1><php the_title(); ?></h1>’ to replace. The entire page’s CSS is:

<?php
get_header(); ?>
	<div id="primary" class="content-area">
		<main id="main" class="site-main" role="main">
		<?php
		while ( have_posts() ) : the_post();
			get_template_part( 'template-parts/content', get_post_format() );
			the_post_navigation();
			// If comments are open or we have at least one comment, load up the comment template.
			if ( comments_open() || get_comments_number() ) :
				comments_template();
			endif;
		endwhile; // End of the loop.
		?>
		</main><!-- #main -->
	</div><!-- #primary -->
<?php
if ( 'post' == get_post_type() ) {
	get_sidebar();
}
get_footer();

Any suggestions on where or how to insert the custom fields tag in a situation like this? I’m running a child theme based on Graph Paper Press’s Optics theme.