Working with jQuery
jQuery is not bundled with your Portal.
If you'd rather use jQuery than work with native, modern JavaScript, you should load it using the m$.loadJS() helper method and run your jQuery-dependent code as a callback.
Log into the API Control Center, go to Manage > Portal, and click Portal Settings. Then add this under the Body JavaScript field under the Inline JavaScript section.
window.addEventListener('portalAfterRender', function () {
// Load jQuery
m$.loadJS('https://code.jquery.com/jquery-3.2.1.min.js', function () {
// Your code goes here...
});
}, false);