Run javascript in lightbox gallery

Question

I am using a gallery plugin and now customize the lightbox appearance for a client.

I want to move the thumbnail div into the caption div and I understand, that I can make it work with jQuery:

    jQuery(".envirabox-thumbs")
      .detach()
      .appendTo(".envirabox-caption-wrap");
   }

But if I have the function run by "onload", the divs for the lightbox haven't loaded yet, and there is no effect.

https://shorturl.at/pwX08

Is there a way to run the script automatically after the lightbox opens?

Thanks a lot, Felix