Drupal 8: How to add third party settings which will be saved automatically.

third_party_settings.php

<?php
/**
 * Implements hook_form_FORM_ID_alter() for 'field_config_edit_form'.
 */
function datalayer_form_field_config_form_alter(&$form, FormStateInterface $form_state, $form_id) {
    $field = $form_state->getFormObject()->getEntity();
    $form['third_party_settings']['datalayer']['label'] = [
      '#type' => 'textfield',
      '#title' => t('DataLayer label'),
      '#default_value' => $field->getThirdPartySetting('datalayer', 'label', $field->get('field_name')),
      '#description' => t('Enter the label you would like in the datalayer output.'),
    ];
  }
}