How to add a custom class to an element in a WordPress post using WP hooks via Snippets?

I'm trying to add a single class (my-class) to an element in a WordPress post (LearnDash Theme). I fount out that it can be done using a hook via add_filter, but no how I could figure it out.

The sample code that I found looks like this:

add_filter( 'body_class', 'custom_body_class' );
/**
 * Add custom field body class(es) to the body classes.
 *
 * It accepts values from a per-page custom field, and only outputs when viewing a singular static Page.
 *
 * @param array $classes Existing body classes.
 * @return array Amended body classes.
 */
function custom_body_class( array $classes ) {
    $new_class = is_page() ? get_post_meta( get_the_ID(), 'body_class', true ) : null;

    if ( $new_class ) {
        $classes[] = $new_class;
    }

    return $classes;
}

In this sample code the class is added to pages only but I need to add it to "posts". I tried is_post() but it didn't work out. I even tried it on a page, but didn't work also.

In my case I want to add a class to an element with ID learndash-page-content.

Existing code:

<div id="learndash-page-content">...</div>

What I'm struggling to do:

<div id="learndash-page-content" class="my-class">...</div>

What do I need to do to get it working?

If that is indeed the syntax of the element, you might want to try string manipulation on the output (which you will capture like this:)

function start_modify_html() {
   ob_start();
}

function end_modify_html() {
   $html = ob_get_clean();
   $html = str_replace( '<div id="learndash-page-content">', '<div id="learndash-page-content" class="my-class">', $html );
   echo $html;
}

add_action( 'wp_head', 'start_modify_html' );
add_action( 'wp_footer', 'end_modify_html' );