With this code snippet, you can show your popup or any other element once user scrolls certain percentage of the page. Scroll up and click on the demo link to see how code is working in real world, clone the project and modify however you want. You can also check how code was added to page.
On line 4, you can select the percentage, which is currently set to 75%. On row19, you can select a div, that you want to affect once the code is triggered.
Copy before tag:
<p> CODE: https://gist.github.com/martinhudobivnik/cc35fd691c07163dedf280105cd5392d.js</p>