muser-8.x-1.x-dev/modules/custom/muser_system/muser_system.links.menu.yml

muser_system.config:
  title: 'Muser settings'
  parent: system.admin_config_system
  description: 'Manage settings for the Muser site.'
  route_name: muser_system.config

muser_system.email_settings:
  title: 'Email settings'
  parent: muser_system.config
  description: 'Manage email settings for the Muser site.'
  route_name: muser_system.email_settings

muser_system.reports:
  title: 'Muser reports'
  parent: system.admin_reports
  description: 'View Muser site reports.'
  route_name: muser_system.reports