Adopt "Add Post Thumbnail Shortcode"

Could you please help with adoption of the “Add Post Thumbnail Shortcode” plugin https://wordpress.org/plugins/add-post-thumbnail-shortcode/

function post_thumbnail_shortcode($atts, $content=”) {
if(!function_exists(‘post_thumbnail_shortcode’))
return;

if(!$atts[‘size’])
$atts[‘size’] = ‘thumbnail’;

return ‘<span class=”post_thumbnail ‘.$atts[‘class’].'”>’.get_the_post_thumbnail(null,$atts[‘size’]).'</span>’;
}

function post_thumbnail($str){
$args = wp_parse_args($str);
echo post_thumbnail_shortcode($args);
}

add_shortcode(‘post_thumbnail’, ‘post_thumbnail_shortcode’);

https://wordpress.org/plugins/external-featured-image/