Create a Popup Menu Item

Any Primary Navigation menu item can be set to open in a popup window instead of redirecting to a new page.

Allow Menu Items to Have a CSS Class

By default WordPress menu items do not have the ability to add a custom CSS class. To enable this, visit your WordPress dashboard. Then click "Appearance" then "Menus". On the top right of the screen click "Screen Options". Once the options are expanded click "CSS Class"

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Add a CSS Class popup to the menu item you wish to create a popup with.
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Optional: If you would like the popup to appear wide when opening add the class popup-wide as well.

Note: Secure Content

Because the popup content is loaded via AJAX be sure the page with the menu item and the popup base page share the same URL protocol. For example if your homepage is http://homepage.com you cannot have a popup with https://homepage.com/my-account as you are not allowed to request secure content from a non-secure origin.