var cookieName = 'codeAndreas_hidePopUp';
var urlPopUp = 'index.php?id=63';

function ShowPopUp() {

    $.ajax({
        url: urlPopUp,
        cache: false
    })
    .done(function( html ) {
        var configuration = ({
            afterOpen: function(event){
            //code here
            
            $( "#DoNotShowPopUpAgain" ).click(function(event) {
                event.preventDefault();
                // setCookie(cookieName, 1);
                setCookie(cookieName, 1, '', '/'); // '/' for whole domain
                var current = $.featherlight.current();
                current.close();
            });
            
            },
            beforeOpen: function(event){
                //code here
            },
            closeOnClick: false
        });

        $.featherlight(html, configuration);
    });

}

function setCookie(name, value, expires, path, domain, secure) {
    var today = new Date();
    today.setTime(today.getTime());
    if (expires) {
        expires = expires * 1000 * 60 * 60 * 24;
    }
    var expires_date = new Date(today.getTime() + (expires));
    document.cookie = name + '=' + escape(value) +
        ((expires) ? ';expires=' + expires_date.toGMTString() : '') + //expires.toGMTString()
        ((path) ? ';path=' + path : '') +
        ((domain) ? ';domain=' + domain : '') +
        ((secure) ? ';secure' : '');
}

function getCookie(name) {
    var start = document.cookie.indexOf(name + "=");
    var len = start + name.length + 1;
    if ((!start) && (name != document.cookie.substring(0, name.length))) {
        return null;
    }
    if (start == -1) return null;
    var end = document.cookie.indexOf(';', len);
    if (end == -1) end = document.cookie.length;
    return unescape(document.cookie.substring(len, end));
}

function deleteCookie(name, path, domain) {
    if (getCookie(name)) document.cookie = name + '=' +
        ((path) ? ';path=' + path : '') +
        ((domain) ? ';domain=' + domain : '') +
        ';expires=Thu, 01-Jan-1970 00:00:01 GMT';
}




$(document).ready(function() {
    
    
    
    console.log('lighboxPopUp.js ready');
    console.log('cookieName:\'' + cookieName + '\'');
    
    var hidePopUp = getCookie(cookieName);
    console.log('hidePopUp:\'' + hidePopUp + '\'');
    
    if (hidePopUp!=1) {
        console.log('ich zeige das PopUp!');
        ShowPopUp();
    }    
    
     
});
lightboxPopUp.js

Create a HTML page popup.html with only this content:

<f:format.raw>{content}</f:format.raw>
<a class="btn btn-primary featherlight-close" href="#">Schließen</a>
<a class="btn btn-primary" href="#" id="DoNotShowPopUpAgain">Schließen & nicht mehr zeigen</a>
popup.html
# SETUP
[page["uid"] == 62]
    # Lightbox example page:
 
    page.includeCSS {
 
        featherlight = //cdn.jsdelivr.net/npm/featherlight@1.7.14/release/featherlight.min.css
        featherlight.external = 1
 
    }
 
    page.includeJSFooter {
        featherlight = //cdn.rawgit.com/noelboss/featherlight/1.7.13/release/featherlight.min.js
        featherlight.external = 1
        lightboxPopUp = fileadmin/downloads/lightboxPopUp.js
    }
 
[global]
 
[page["uid"] == 63]
    # page-ID with popUp-content
    page {
        10.file.override = fileadmin/templates/website/popup.html
        config.disableAllHeaderCode = 1
    }
[global]
setup
[page["uid"] == 37]
    page {
        10.templateName.override = Popup
        config.disableAllHeaderCode = 1
    }
[global]
mit Sitepackage etc.