Alter field instance settings form on admin/structure/types/manage/<content type>/fields/node.<content type>.<fieldname>/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(&$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) { $form['enabled'] = array( '#type' => 'checkbox', '#title' => t('Enable'), '#description' => t('If enabled then this module will alter the field settings.'), '#default_value' => $form['enabled'], ); }