Issue #3250228 by andrewbelcher: Expose overriding of settings

      '#default_value' => $this->entity->getDescription(),

    ];

    // Create an arbitrary entity object (used by the 'default value' widget).

    $ids = (object) [

      'entity_type' => $this->entity->getTargetEntityTypeId(),

      'bundle' => $this->entity->getTargetBundle(),

      'entity_id' => NULL,

    ];

    $form['#entity'] = _field_create_entity_from_ids($ids);

    $items = $form['#entity']->get($this->entity->getName());

    $item = $items->first() ?: $items->appendItem();

    // Add field settings for the field type and a container for third party

    // settings that modules can add to via hook_form_FORM_ID_alter().

    $form['settings'] = [

      '#tree' => TRUE,

      '#weight' => 10,

    ];

    $form['settings'] += $item->fieldSettingsForm($form, $form_state);

    return $form;

  }