I need to display country name in a select filed and display its city names in another select field according to selection change committed on country name field. i am using ajax for selection change event. This is what i have done so far. now i need to fetch city names from second api corresponding to the country name in the select field.

public function getFormId() {  return 'newmoduleform_ajax_form'; }  public function buildForm(array $form, FormStateInterface $form_state) {  // First API Call to populate country select field    $result = [];  $country_options = [];  $client = Drupal::httpClient();  // Fetch API values.   $request = $client->request('GET','https://restcountries.com/v2/all');  $result = Json::decode($request->getBody()->getContents());   foreach ($result as $item) {    $country_options[] = $item['name'];   }      if (empty($form_state->getValue('country_dropdown'))) { // Use a default value.   $selected_option = key($country_options);   }   else {   // Get the value if it already exists.   $selected_option = $form_state->getValue('country_dropdown');    }  $form['option_country_fieldset'] = [   '#type' => 'fieldset', ];  $form['option_country_fieldset']['country_dropdown'] = [   '#type' => 'select',   '#title' => $this->t('country'),   '#options' => $country_options,    // Bind an Ajax callback to the element.   '#ajax' => [     'callback' => '::instrumentDropdownCallback',     'wrapper' => 'state-fieldset-container',     'event' => 'change',   ], ];  $form['select_fieldset_container'] = [   '#type' => 'container',   '#attributes' => ['id' => 'state-fieldset-container'], ];  $form['select_fieldset_container']['select_fieldset'] = [   '#type' => 'fieldset',    ];  $form['select_fieldset_container']['select_fieldset']['select_dropdown'] = [   '#type' => 'select',   '#title' =>  $this->t('Country Name-'). ' ' . $country_options[$selected_option],   '#options' => static::getSecondDropdownOptions(), ]; return $form; }   public function submitForm(array &$form, FormStateInterface $form_state) {   }  public function instrumentDropdownCallback(array $form, FormStateInterface $form_state)  {  return $form['select_fieldset_container']; }  public static function getSecondDropdownOptions() { [![second api that have city name][1]][1] $result_second = []; $city_options = []; $client = Drupal::httpClient(); // Fetch API values.  $request = $client->request('GET','https://countriesnow.space/api/v0.1/countries'); $result_second = Json::decode($request->getBody()->getContents());  foreach ($result_second[data] as $item_second) {     // what to do here  }   // what to do here }