drupal_set_message
Displaying 1 - 1 of 1Success Message After Submitting Form
In building a custom form you must keep in mind a few issues:
elements, functionality, and response for best user experience
DrupalVIP technical notebook & support
Code Snippet
public function submitForm(array &$form, FormStateInterface $form_state) {
$this->logger('user')->notice('Deleted %ip', ['%ip' => $this->banIp,]);
$this->messenger()->addStatus($this->t('The IP address %ip was deleted.', [ '%ip' => $this->banIp, ]));
$form_state->setRedirectUrl($this->getCancelUrl());
}