You can either add the script tag in the footer.php or in the header.php of the theme you are working. You can then add the html in your footer or a file that that you want to have the modal to show up. The benefit of adding it to the footer is because footer is likely being added on all pages so the modal is available on all pages. Then in the footer.php add the following line. Everything should work.
var quickviews = bulmaQuickview.attach();