in appearance → menus
go to screen options and enable css classes:
Screenshot 2023-03-02 at 15.49.32.png
then add a css class to your menupoint like: scrollup
then add this JS to lay options → custom html & css → custom <head> content :
jQuery(document).on("click", ".scrollup", function(e){ e.stopPropagation(); e.preventDefault(); jQuery('html, body').animate({scrollTop: 0}, 600, 'easeOutQuint'); })