I want to add a menu item on wordpress that triggers

I want to add a menu item on wordpress that triggers the popup, I've seen the functionality where a custom css selector can be used to trigger them, and works fine when I can add simply a link with a custom css class or id and then just add ".myclass" on the selector area for the trigger, but in wordpress menus, I can only add custom css classes on an element, but it actually adds those classes to the parent li of the link in the menu, so I tried something like ".myclass > a" on the selector and didn't work.

In a desperate attempt, I tried with a custom javascript added through functions.php on the footer to add the custom class (guestlist-popup on the screenshot) on the actual element instead of just the parent li but it didn't work either (I suppose the javascript change happens after the js used in the plugin). What would you advice to be able to accomplish making a menu item the trigger for the popup, can I add a certain javascript function on it that triggers onclick or something?

Please advice soon, it's a bit urgent