Woocoomerce: Open pop-up upon clicking the terms checkbox

I've started to be a bit desperate here. So I have a check-box in the terms area of my woocommerce checkout to agree that you read our T&C. However, we are offering services that ABSOLUTELY require people to read our terms because they include health hazard information. Obviously, legally we will be fine by offering the terms and having people agree that they read them.

What I want to achieve, though, is that a pop-up opens, when that check-box is clicked. It does not have to be fancy or styled, but since this is in woocommerce I am VERY unsure about where to place what.

A simple <input type="checkbox" id="xxx"> type and javascript was my first guess, but I do not know where to put what in this case since Woocommerce seems to be a rather closed ecosystem, so neither did I find where to place my JS (too many different phps?) nor where to link the checkbox. I was so lost.

Maybe you guys have a different approach or know where to put what.

Thanks!