Add the 'mpopup' class to a menu item in a custom menu to open the target in a popup Window.
This is a fork of my similarly named Alligator Popup Plugin. Unlike the original plugin, this one enables you to open the target of a menu item of your WordPress custom menu in a popup window.
Add the ‘mpopup’ class to a menu item in a custom menu to open the target in a popup Window.
Enables you to specify that a menu item on your WordPress custom menu will open in a new popup window. An admin page allows you can control the size of the popup window and whether it has scrollbars.
You will find the Menu Popup Settings Page at Settings => Menu Popup
Then edit your custom menu at Appearance => Menus
* Enable the CSS Classes option in the Screen Options pull-down panel on the menu editor page.
When the menu item is clicked, the link should now open in a popup window.
Install the same as any other plugin
see: https://codex.wordpress.org/Managing_Plugins#Installing_Plugins
You’ll find it at settings => Menu Popup
If your menu item editor is not showing the ‘CSS Classes’ field, you can enable this by going into ‘screen options’ and selecting the ‘CSS Classes’ checkbox.
You can use my Alligator Popup plugin for that
It works for me, but all sites are different. Post a support request and I’ll try to help you get it working