prepopulate-8.x-2.3/tests/modules/prepopulate_test/prepopulate_test.routing.yml

prepopulate_test.test_form:
  path: '/prepopulate_test/form'
  defaults:
    _form: '\Drupal\prepopulate_test\Form\PrepopulateTestForm'
    _title: 'Pre-populate Test Form'
  requirements:
    _access: 'TRUE'