Alter field instance settings form on admin/structure/types/manage//fields/node../storage I'm trying to figure out how to add an extra field to content type field settings page.

Basically I want to alter field instance settings form on admin/structure/types/manage//fields/node../storage .

I'm going with hook_form_FORM_ID_alter for FORM_ID field_config_edit_form.
The new form element appears on the form but after submit I'm not able to see the value.

Am I missing something? Is an extra form submit function required other than the default one to submit my new value?

Here is my snippet - 

/**
 * Implements hook_form_FORM_ID_alter().
 */
function mymod_form_field_storage_config_edit_form_alter(&amp;$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {

  $form['enabled'] = array(
      '#type' =&gt; 'checkbox',
      '#title' =&gt; t('Enable'),
      '#description' =&gt; t('If enabled then this module will alter the field settings.'),
      '#default_value' =&gt; $form['enabled'],
  );
}
Here is an example -

/**
 * Implements hook_form_FORM_ID_alter().
 */
function mymod_form_field_storage_config_edit_form_alter(&amp;$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {

  $form['enabled'] = array(
      '#type' =&gt; 'checkbox',
      '#title' =&gt; t('Enable'),
      '#description' =&gt; t('If enabled then this module will alter the field settings.'),
      '#default_value' =&gt; $form['enabled'],
  );

  $form['#entity_builders'][] = '_my_additional_form_field_storage_submit';
}

/**
 * Entity builder.
 */
function _my_additional_form_field_storage_submit($entity_type, Drupal\field\Entity\FieldStorageConfig $fieldStorageConfig, &amp;$form, \Drupal\Core\Form\FormStateInterface $form_state) {

  if ($form_state-&gt;getValue('enabled')) {
    $fieldStorageConfig-&gt;setThirdPartySetting('my_additional_settings', 'enabled', $form_state-&gt;getValue('enabled'));
  }
  return true;
}