Display value of custom field at the beginning of a post

You need to prepend value stored in _subtitle to the content of the post so that it works. Please place following code in your theme’s functions.php file and it shall work. Please let me know how it goes so that I may assist further:

add_filter( 'the_content', 'prepend_subtitle_to_content' );
function prepend_subtitle_to_content( $content ){
    $subtitle = get_post_meta( get_the_ID(), '_subtitle', true );
    if ( !empty( $subtitle ) ) {
        return esc_html( $subtitle ) . $content ;
   }
   return $content;
}