levmyshkin/drupalbook.module

## drupalbook.module
<?php

/**
 * Implements hook_form_alter().
 */
function drupalbook_form_alter(&$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {
  if ($form_id == 'drupalbook_admin_settings') {
    $form['drupalbook_api_key']['#attributes']['placeholder'] = 'API key';
    $form['drupalbook_api_client_id']['#attributes']['placeholder'] = 'API client ID';

    $form['#validate'][] = 'drupalbook_settings_validate';
    $form['#submit'][] = 'drupalbook_settings_submit';
  }
}

/**
 * Custom validation callback.
 */
function drupalbook_settings_validate(&$form, \Drupal\Core\Form\FormStateInterface $form_state) {
  if (strpos($form_state->getValue('drupalbook_api_key'), 'google') === FALSE) {
    $form_state->setErrorByName('drupalbook_api_key', t('API Key must start from "google".'));
  }
}

/**
 * Custom submit callback.
 */
function drupalbook_settings_submit(&$form, \Drupal\Core\Form\FormStateInterface $form_state) {
  // drupal_set_message is deprecated
  // drupal_set_message(t('An error occurred and processing did not complete.'));

  \Drupal::messenger()->addStatus(
    t(htmlentities('Insert API key in your <head> tag:  <script async defer src="https://maps.googleapis.com/maps/api/js?key=YOUR_API_KEY&callback=initMap" type="text/javascript"></script>.'))
  );
}
	<?php

	/**
	* Implements hook_form_alter().
	*/
	function drupalbook_form_alter(&$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {
	if ($form_id == 'drupalbook_admin_settings') {
	$form['drupalbook_api_key']['#attributes']['placeholder'] = 'API key';
	$form['drupalbook_api_client_id']['#attributes']['placeholder'] = 'API client ID';

	$form['#validate'][] = 'drupalbook_settings_validate';
	$form['#submit'][] = 'drupalbook_settings_submit';
	}
	}

	/**
	* Custom validation callback.
	*/
	function drupalbook_settings_validate(&$form, \Drupal\Core\Form\FormStateInterface $form_state) {
	if (strpos($form_state->getValue('drupalbook_api_key'), 'google') === FALSE) {
	$form_state->setErrorByName('drupalbook_api_key', t('API Key must start from "google".'));
	}
	}

	/**
	* Custom submit callback.
	*/
	function drupalbook_settings_submit(&$form, \Drupal\Core\Form\FormStateInterface $form_state) {
	// drupal_set_message is deprecated
	// drupal_set_message(t('An error occurred and processing did not complete.'));

	\Drupal::messenger()->addStatus(
	t(htmlentities('Insert API key in your <head> tag: <script async defer src="https://maps.googleapis.com/maps/api/js?key=YOUR_API_KEY&callback=initMap" type="text/javascript"></script>.'))
	);
	}