Creating my own Sub-Menu / Parent in the Admin/Config menu
Just thought it would be easier to show a basic example that has both parent and submenu item in the mymodule.links.menu.yml. 

Right now both links goto the same place (as I had to pick a default for the parent item, since I didn't have a admin page that lists all my custom links yet ... and also since having "route_name: &lt;nolink&gt;" did not work for the parent menu item even though some peeps said it worked for them [Note: using Drupal 8.6.12]):

mymodule.parent:
  title: 'My MainMenu Link'
  parent: system.admin_config
  description: 'Custom main config menu item under the Admin-&gt;Configuration menu.'
  route_name: mymodule.form
  weight: 100

mymodule.parent_child:
  title: 'My Submenu Link'
  parent: mymodule.parent
  description: 'This is the sub-menu item for my custom config form.'
  route_name: mymodule.form
  weight: 101