WebPlus X6: Lightbox Button Close

Hi... I'm trying to put a "close" button on my youtube video which is inside a lightbox. I located something online and have done as suggested.... and I don't have an "X" to close the video in the lightbox (see ButtonClose)

"ButtonClose

Right click button>Actions>Add>Custom. The Event is Click, and paste the following line in the Custom script box"

I didn't see any kind of script to go in the header for this particular action, should there be one?

Thanks
Leah