Our new license plate field type could be added to an entity type, but there would be no way users could use it. For this, we will need at least a widget. A given field type can work, however, with multiple widgets. So, let's create that default license plate widget plugin we referenced in the annotation of the field type, which belongs in the Plugin/Field/FieldWidget namespace of our module:
namespace Drupal\license_plate\Plugin\Field\FieldWidget;
use Drupal\Core\StringTranslation\StringTranslationTrait;
/**
* Plugin implementation of the 'default_license_plate_widget' widget.
*
* @FieldWidget(
* id = "default_license_plate_widget",
* label = @Translation("Default license plate widget"),
* field_types = {
* "license_plate"
* }
* )
*/
class DefaultLicensePlateWidget extends WidgetBase...