Adding Subtitle beneath Post Titles

Hello,

I'm attempting to add a subtitle field beneath post titles.

My first thought was to use the WordPress Plugin:
https://wordpress.org/plugins/wp-subtitle/

And then add their code as and Element Hook to appear beneath a post title (with PHP activated in the Element):

PHP Code:
do_action( 'plugins/wp_subtitle/the_subtitle', array(
'before' => '<p class="subtitle">',
'after' => '</p>',
'post_id' => get_the_ID(),
'default_value' => ''
) );

Unfortunately adding the above element/hook/PHP code didn't work.

What are your suggestions?

Thank you,

Tal White