Custom HTML in Promotional Popup Box

Hey all, our email marketing platform (Campaign Monitor) creates its own proprietary sign-up forms that can be embedded on websites with generated HTML. 

I would love to have this form embedded in a pop-up that shows up on our website after visitors are there for a few seconds (and then doesn't reappear if the page is reloaded and the user closes out the window). Basically, I would love the functionality of the Squarespace's existing built-in pop-up feature but to add in my own custom HTML form instead of being forced to use their internal form.

I've done a bunch of research on this, and can't find good free options or custom CSS/HTML for this, so I'm hoping somebody here can help.

Can you send embed code? I can try testing it on my site