config.php

<?php

/**
 * Hooks
 */
$GLOBALS['TL_HOOKS']['processFormData'][]   = array('Hooks', 'processFormData');

Hooks.php

<?php

use NotificationCenter\Model\Notification;

class Hooks
{
	public function processFormData($post, $form, $files)
	{
		$notificationIds = (array) unserialize($form['notifications']);

		foreach ($notificationIds as $notificationId) {
			if ($notificationId) {
				$tokens = [];

				foreach ($post as $key => $value) {
					$tokens['form_' . $key] = $value;
				}

				/** @var Notification $notification */
				$notification = Notification::findByPk($notificationId);
				if (null !== $notification) {
					$notification->send($tokens);
				}
			}
		}
	}
}

tl_form.php

<?php

$GLOBALS['TL_DCA']['tl_form']['fields']['notifications'] = array(
	'inputType' => 'checkbox',
	'label'     => array(
		'Benachrichtigungen',
		'Wählen Sie hier aus, welche Benachrichtigungen gestartet werden sollen.'
	),
	'foreignKey'   => 'tl_nc_notification.title',
	'eval'      => array(
		'multiple' => true,
		'tl_class' => 'clr'
	),
	'sql'       => 'text NULL'
);