A menu link can be linked in WordPress only with certain elements or with external pages. If a menu link should open e.g. a modal, this is usually more complicated. A remedy for this is a jQuery script.
Solution: You just change the ID of the desired menu link and define the code that should be executed when clicking on this menu item.
jQuery(document).ready(function(){ jQuery("#menu-item-50").bind( "click", function() { //Here is the code you want }); });