How do I get my custom route to a custom menu link What I am trying to do is:
I want my custom form to show up on a custom path under

    /admin/config/content/mymodule


My routing.yml looks like this

    mymodule.mymoduleform:
      path: '/admin/config/content/mymodule'
      defaults:
        _form: '\Drupal\mymodule\Form\Mymoduleform'
        _title: 'Mymodule'
      requirements:
        _access: 'TRUE'


And my menu.links.yml like that

    mymodule.admin:
      title: 'Mymodule'
      description: 'Manage mymodule'
      parent: system.admin_confifg_content
      route_name: mymodule.mymoduleform
      weight: 100


Pityfully the doku is very thin about that hope you guys have a clue bout this :)

Or maybe i have to figure out how I get the right parent.
I want that menu link displayed under "Content Authoring"
Have a look at filter.links.menu from core. There you can see how the filter module is placing its link under 'Content Authoring'. You can do is the same way.