How do I add a menu item to the main navigation from a custom module in Drupal 8? For my custom Drupal 8 module I want to add a link to the default main navigation menu of the site.

What parent ID do I have to give to insert my menu item into the main menu?

I currently have the following config in my_module.links.menu.yml:

entity.sss_topic.collection:
  title: 'Topic: Listing'
  route_name: entity.sss_topic.collection
  description: 'List Topic'
  parent: main


But that only adds a menu item into the "Tools" menu (the same effect as specifying no parent whatsoever).
However, parent: system.admin_structure does work correctly.

Do I have the wrong parent ID? Or maybe a bug?
You are missing the "menu_name" key as said on : https://www.drupal.org/node/2122241

this example should work : 

demo.demo:
  title: Demo Link
  description: 'This is a demo link'
  parent: main
  route_name: demo.demo
  menu_name: main