Programatically define Menu links I want to show Login link in the Menu navigation, only for guest User.

clinical_trial.login:
  title: Login
  description: 'Login'
  parent: main
  menu_name: main
  route_name: user.login


This is my Custom Menu links code.

What to add?
If you are creating a menu link from a custom module, you can check this 

https://www.drupal.org/docs/8/api/routing-system/structure-of-routes

Quoting the documentation:

requirements (required): Determines what must be met in order to grant access to the route. 
    _role: A specific Drupal role, eg. 'anonymous'. 


Alternatively you can use this module: https://www.drupal.org/project/menu_item_visibility
"It exposes configurable and extendable visibility settings for menu links."