How to create a pop-up modal on the community?

I think you need to do that via Third Party Scripts. 

I did a pop up modal recently on our registration page and that’s where I put the script. 

We have done this with Third Party Scripts as well. For a while, we had a pop-up when someone had sat on one of our Product Documentation articles for longer than 10 seconds to ask if they were finding what they needed via an embedded Typeform. I think this is what we used (cleaned up with placeholders). 
 

<script>
// Wait for the DOM content to be fully loaded
document.addEventListener("DOMContentLoaded", function() {
    // Use setTimeout to delay execution by [DELAY IN MILLISECONDS]
    setTimeout(function() {
        // Check if the breadcrumb element with the title "[PAGE OR SECTION NAME]" exists on the page
        if (document.querySelector('[data-preact="widget-breadcrumb/Breadcrumb"][data-props*="[PAGE OR SECTION NAME]"]')) {
            // If the breadcrumb exists, execute the code
            var divElement = document.createElement('div');
            // Add your custom embed attribute here (example shown for Typeform)
            divElement.setAttribute('[EMBED-ATTRIBUTE-NAME]', '[YOUR-EMBED-ID]');
            document.body.appendChild(divElement);

            // Load your external script (example shown for Typeform)
            var scriptElement = document.createElement('script');
            scriptElement.src = '[EMBED-SCRIPT-URL]'; // e.g. https://embed.typeform.com/next/embed.js
            document.body.appendChild(scriptElement);
        }
    }, [DELAY IN MILLISECONDS]); // Example: 10000 = 10 seconds
});
</script>

Thank you so much! ​@DannyPancratz ​@atwhite