Reply To: How to add custom link with JS on header menu under dropdown?

Hello,

You just add a custom link and insert your URL inside, then you need to add two custom links as sub-menu items to this first custom link. You will get your custom links with the drop-down without any additional JS. Then if you need help, contact I will help to customize the styles.

Check Creating Multi-level Menus here https://codex.wordpress.org/WordPress_Menu_User_Guide

Best Regards