A menu normally contains all categories of your website. And it's necessary to add category link to your menu so that customers can quickly navigate to any category they want.
In this post, we'll show you how to add category link to a menu created by Ninja Menus, a simple and intuitive Magento 2 mega menu extension.
https://www.magezon.com/magento-2-mega-menu.html

1. Navigate to Content > Elements > Menus to view all created menus in a grid. In Action column, click Select > Edit of the menu you want to add category link.
2. Open Menu Builder tab. Hover over the menu item you want to add category link and click the pencil on the bar to edit the menu item.
3. In General tab, choose Category Link for link Type.
4. Enter category name or ID in Category field to specify the category you want to link to.