How can I create a menu link that shows only to authenticated users? How can I create an user account menu link that shows up only to registered/authenticated users in Drupal 8? I do not want anonymous users to see it.

For example, I want to have "add node" menu link.

( When anonymous users come to drupal sites they just see "login" on user account menu . When register users come to drupal sites they see "my account" and "logout" . I want show anonymous just "login" and for register users "my account","add node" and " logout .  )
Long story short, you create 2 different Menu versions and give each version different visibility rules. 


All you gotta do is configure your menu link block



Then in Roles, check mark ☑ Authenticated users and  you may want to also ☑ Administrator



Now create another Menu (/admin/structure/menu) which does not have the "add node" link. Then go to blocks (/admin/structure/block) in Primary Menu (Assuming you're using Bartik, may have a different name if you're using a custom theme) click add, and add the Menu you just created and configure it. 

This time for Roles, only check mark ☑ Anonymous


  PS: In pages you can restrict so it only shows up in certain pages.