public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state)
  {

// come code here ....

    // TODO: Implement formElement() method.
    $element['country_id'] = [
      '#type' => 'select',
      '#title' => t('Select a country'),
      '#options' => PalomGeo::getCountryList(),
      '#default_value' => $country_id,
      '#ajax' => [
        'callback' => '::changeCountry',
      ]
    ];

    $regions = PalomGeo::getRegionList($country_id);

    if ($regions!=[]){
      $element['region_id'] = [
        '#type' => 'select',
        '#title' => t('Select a region'),
        '#options' => $regions,
        '#default_value' => $region_id,
      ];
    }

    return $element;
  }

// Some code here

  public function changeCountry(array &$form, FormStateInterface $form_state){
    return $form['region_id'];
  }