Howdy @painlessseo!
It depends on what methods your GDPR Plugin has available. Generally, Content Mask is intended to grab the entirety of content from “xyz website” and display it on your domain (such as with landing pages, that’s actually what I built it for!) The thing with that, is that it fetches the entire page, as-is, and doesn’t really bring much of anything from the main site with Content Mask installed.
That said, there are a few things you can do, depending on how your GDPR plugin is set up. If there’s a way for you to get the JavaScript and CSS for the popup, you can put them in the Header or Footer scripts section (either in the individual page in the Content Mask metabox, or universally on the Content Mask > Scripts & Styles admin page).
If there’s a PHP function, template tag, or shortcode, you can add it to the content_mask_iframe_header or content_mask_iframe_footer, but that’d require the download method to be changed to iframe (I’m not sure if that’ll play nice with your current landing page) as well as some PHP programming.
There isn’t currently a great way to hook shortcodes or widgets or anything into the Download method. I had issues in the past with plugins messing with the Download method because when you grab standard WordPress functions that those things are usually tied to, they drop in other scripts and styles from other plugins and almost always break the page because of conflicting code.
Sorry for the rambling, it’s a long way of saying the easiest is to see if you can get the Scripts and Styles from your GDPR plugin and put them in the Header & Footer scripts section of Content Mask. Something like:
<script src="url/to/any/scripts/your/plugin/requires.js"></script>
<script>my_plugins_gdpr_popup();</script>
<link rel="stylesheet" href="url/to/any/styles/style.css">
<!-- or -->
<style>
/* your stylesheets' content here */
.my_gdpr_popup {
display: block;
/* etc */
}
</style>
The last alternative I could think of would be to make a simple GDPR Popup like one of these: https://codepen.io/vcoppolecchia/pen/gzQMXB (not mine, credit to @vcoppolecchia) and put the HTML, JS, and CSS in the Header & Footer scripts section.
Sorry I don’t have an easy one-and-done answer for you!
