Adding Javascript into Main Navigation item

Site URL: http://www.mightyyoganh.com/

I'm a designer and not at all a developer so I need some help with my client's request. She would like to add a javascript widget as a main navigation item but there is no option to add code to the main nav. The widget is a pop up that would allow the user to sign up for classes through MindBody instead of linking offsite. 

She provided an example site that has accomplished this: https://bodywisdomstudio.net/ (Login/Register button in main nav) but I'm not savvy enough to parse through the code.

Any suggestions?