I would like to remove the 'cross' sign from a specific pop-up

I would like to remove the cross sign from one of my popups. Not all the popups, just one of the them.
I've seen this custom code in a forum
.wdpu-msg a.wdpu-close {
display: none !important;
}
And I need to tailor it to a specific popup but I don't know how to get the pop-ups ID code.
Also, that code alone does not seem to be working (I assume that code should take the cross away from all popups which it isn't)
I've created a 'test' popup on this page - http://www.katieo.com/growth-hub/
Thanks in advance, your help is appreciated!
Katie