FormFactory.php

<?php
declare(strict_types=1);
namespace Wwwision\Test;

use Neos\Flow\Validation\Validator\NotEmptyValidator;
use Neos\Form\Core\Model\FormDefinition;
use Neos\Form\Factory\AbstractFormFactory;

final class FormFactory extends AbstractFormFactory
{

    public function build(array $factorySpecificConfiguration, $presetName) {
        $formConfiguration = $this->getPresetConfiguration($presetName);
        $form = new FormDefinition('testform', $formConfiguration);

        $page1 = $form->createPage('page1');

        $password = $page1->createElement('password', 'Neos.Form:Password');
        $password->addValidator(new NotEmptyValidator());
        $password->setLabel('Password');

        $confirmation = $page1->createElement('confirmation', 'Wwwision.Test:PasswordConfirmation');
        $confirmation->setLabel('Confirm');

        return $form;
    }
}

PasswordConfirmation.php

<?php
declare(strict_types=1);
namespace Wwwision\Test\Form\Elements;

use Neos\Form\Core\Model\AbstractFormElement;
use Neos\Form\Core\Runtime\FormRuntime;

final class PasswordConfirmation extends AbstractFormElement
{
    public function onSubmit(FormRuntime $formRuntime, &$elementValue)
    {
        \Neos\Flow\var_dump($formRuntime->getFormState()->getFormValues(), 'form values');
    }
}

Settings.yaml

Neos:
  Form:
    presets:
      default:
        formElementTypes:
          'Wwwision.Test:PasswordConfirmation':
            superTypes:
              'Neos.Form:FormElement': true
            implementationClassName: Wwwision\Test\Form\Elements\PasswordConfirmation