How to add onFinish attribute?

I load iframes using shadowbox, and I want the loading animation to keep spinning until the iframe is fully loaded.

I found this code, tried to add it to the footer of the theme but it doesn’t work. Any other way to make it work?

Shadowbox.init({
            onFinish: shadowboxFinish
        });

function shadowboxFinish() {

            $("#sb-loading").show();

            $("#sb-player").load(function () {

                $("#sb-loading").hide();
             });
        }

Thanks a lot,
Ciprian

https://wordpress.org/plugins/shadowbox-js/