Customizing a Carousel-portfolio-section with a code for a video lightbox button

Hi everyone, 

I need help to costumize the buttons in the carousel-portfolio-section so that they can open a lightbox.

I would like to use the carousel section to show short films posters, and to use the buttons under each picture to open a lightbox with the corresponding trailer (hosted on YouTube). 

I found a great tutorial with a free code (link below) that shows how to create a button to open a video-lightbox. I tried to put this code (with the 3 options of the tutorial) in the button costum area, and it seemed to work until I saved changes ; after that, it doesn't work anymore and the code appears in the button… then when I click on it, it brings me to the Error Page.

Here are some screenshots of the problem…

Does anyone know how to fix it ? how to costumize a button content in a designed section ?

Thank you for your help !

Link toward the tutorial page : https://www.kenwestphal.io/spacetutorials/squarespace-lightbox-video/

Link toward the related tutorial video : https://www.youtube.com/watch?v=dcGqrujn_B8

Thank you very much !