Nav is 2nd block, looking to make it pop up when scrolled down to

I have a main splash screen when the user visits the site. I have the navigation bar hidden under it, so it will fade in once the user scrolls down, revealing the rest of the site. How do I go about adding my JS to make this happen in Blocs? Or is there another way this needs to be done with this software?

I already have the couple lines of JS needed to perform the task, just not sure how to make it work with blocs.