Adding a menu-tab link is quite easy. Here i am going to add a “who’s Online” menu link link to the menu-tab under “admin/content” using views.
“Who's online block” is a default view in drupal 8 which has a block display. Clone the block into a page and provide the path as “admin/content/xxxxxx”. Now click “menu” in the view and choose menu type as “Menu tab”. Name the menu at “Menu link title“ and keep the parent as “-- Administration” and save it. Now clear the cache and visit “admin/content” where a new Menu tab link will be added to list the online users.