Do you want a modal that is activated when clicked on, or a modal that’s activated after a certain time on a site?
If it’s the former, check out the following tutorial: https://www.w3schools.com/howto/howto_css_modals.asp
Add the HTML and script either in the source code of a WYSIWYG post (if it’s just for one post) or add it to a partial/page if it shows up on multiple pages. Add the CSS to the Custom CSS module in the GUI or main.css in your site files.
