examples-3.x-dev/modules/config_simple_example/config/schema/config_simple_example.schema.yml

config_simple_example.settings:
  # A config_object is a configuration object which can store fields.
  # You have to map all field types the object stores: string, boolean, etc.
  # The "text" field type is special because it's translatable by default.
  # Checkout how to define schema and all available data types in:
  # https://www.drupal.org/docs/drupal-apis/configuration-api/configuration-schemametadata
  # Direct link to cheat sheet:
  # https://www.drupal.org/files/ConfigSchemaCheatSheet2.0.pdf
  # Use Configuration Inspector to validate schema files:
  # https://drupal.org/project/config_inspector
  type: config_object
  label: 'Example config'
  mapping:
    message:
      type: text
      label: 'Message'