Popup Pro Scrolling on Mobile

[Lupylu]

Hi, I have been trying to set up a responsive size chart using Popup pro, after following the tutorial I have the size chart pop up working on both Full size screens & mobiles. However I would like to be able to scroll vertically on a mobile, as well as on a full sized screen. Currently on a mobile the background scrolls, instead of the popup.

I know this is possible, as this module has the feature I require: http://addons.prestashop.com/en/18092-ilm-size-chart.html

I have found this script that should work, but not sure where I need to place it within the module files: 

<script>

var $html = $(document.documentElement); var $modal = $('.my-custom-modal');

function showModal() { $html.css('overflow', 'hidden'); $modal.show().css('overflow', 'auto'); }

function hideModal() { $html.css('overflow', ''); $modal.hide(); }

$('.modal-open-button').on('click', showModal); $('.modal-close-button').on('click', hideModal);

</script>

Any help would be gratefully received.

Many Thanks