Hello,
Basel Theme provides the options extending the default WordPress menu functions making the menu more flexible and powerful by means of megamenu blocks. Megamenu blocks are WP Bakery page builder templates, containing Extra menu elements to make sub-menu better arranged and looking nicer. Besides Extra menu elements, megamenu blocks can contain images, text and other elements from WP Bakey page builder chart.
As soon as Megamenu blocks are ready, they may be added to menu items in Appearance -> Menu -> Select menu -> enter menu item -> HTML Block dropdown in the Custom fields [for theme] which appear only after the menu has been saved. You should insert the HTML shortcode into the Description field. https://xtemos.com/docs/basel/create-menu/
Best Regards