Copy and Paste the following code for creating a custom popup with html/css and js or download the file click here
var btnElement = document.getElementById("popupbtn"); var modalElement = document.getElementById("kmodal");
btnElement.onclick = function() { modalElement.style.display = "block"; }
var closeElement = document.getElementById("kmodal_close"); var footercloseElement = document.getElementById("footer_kmodal_close"); closeElement.onclick = function() { modalElement.style.display = "none"; }
footercloseElement.onclick = function() { modalElement.style.display = "none"; }