If not logged in show popup for login

I have a question about how t use this plugin together with the Login With Ajax plugin.

I only want registered users to be able to add favorite posts. What I want to achieve is that if a users click on the “add to favorite” link when they are not logged in, the popup from Login with ajax will appear.

I have a feelin I have to use it in this piece of code of wp-favorite-posts.php:

function wpfp_add_favorite($post_id = "") {
    if ( empty($post_id) ) $post_id = $_REQUEST['postid'];
    if (wpfp_get_option('opt_only_registered') && !is_user_logged_in() ) {
    //this is not working:
 // -> echo do_shortcode('[lwa]'); OR
// -> login_with_ajax('registration=1');

        wpfp_die_or_go(wpfp_get_option('text_only_registered') );
        return false;
    }

Hope somone can help me with this!

https://www.ads-software.com/plugins/wp-favorite-posts/