examples-3.x-dev/modules/content_entity_example/content_entity_example.links.action.yml

content_entity_example.contact_add:

  # The route which will be invoked when the link is clicked.
  route_name: content_entity_example.contact_add
  title: 'Add contact'

  # The pages where the link appears, indicated by their route names.
  appears_on:
    - entity.content_entity_example_contact.collection
    - entity.content_entity_example_contact.canonical