Please add below code to bottom of your functions.php, I have replaced with your menu item ID
function lidget_popup_link() {
?>
<script type="text/javascript">
jQuery(window).load(function() {
var width = "680", /* Enter popup window width here */
height = "480", /* Enter popup window height here */
menuItem = "menu-item-140", /* Enter menu item ID */
target = "#"+menuItem+" a",
hrefVal = jQuery(target).attr("href")
;
jQuery(target).attr({
"href" : "#",
"onclick" : "window.open('"+hrefVal+"','','width="+width+",height="+height+",scrollbars=no,resizable=no,location=no,menubar=no,toolbar=no')"
});
});
</script>
<?php
}
add_action('wp_head', 'lidget_popup_link');
function lidget_call_jquery() {
wp_enqueue_script('jquery');
}
add_action('wp_enqueue_scripts', 'lidget_call_jquery');
?>
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OUTPUT (contact page open in pop up):