Enable lightbox with custom URL

Thanks for the custom URL plugin! I hope that this question is not redundant, I couldn’t find it in the forum.

I am creating an image gallery and some of my photos will use custom links to another image. What I want is to launch the gallery lightbox when I click on the image with the custom URL. Right now, with the plugin, the image opens in another page, not in the lightbox (see image 6 in the gallery on the page that I linked to). When I open the lightbox from another image and flip through the gallery the custom URL appears as I would like it to… but not when I click that image to open the lightbox.

Is there a way to have the custom URL open in the lightbox?

Thanks for your help!

The page I need help with: [log in to see the link]