Adding additional fields to TYPO3 extension seminars

I try to add additional fields to the extension seminars (5.7.0) in TYPO3 10.4.34 but they don't show up in the single view in frontend.

What I did so far:

1. I created the extension my_seminar with the new field precondition. The field is shown in the backend.

2. I extended the seminars event model in the file my_seminar/Classes/Domain/Model/Event.php:

namespace MyVendor\MySeminar\Domain\Model;

class Event extends \OliverKlee\Seminars\Model\Event
{
     /**
     * precondition
     *
     * @var string
     */
    protected $precondition = '';


     /**
     * Returns the precondition
     *
     * @return string $precondition
     */
    public function getPrecondition()
    {
        return $this->precondition;
    }

    /**
     * Sets the precondition
     *
     * @param string $precondition
     * @return void
     */
    public function setPrecondition($precondition)
    {
        $this->precondition = $precondition;
    }
}

3. I created the hook EventSingleView in the file my_seminar/Classes/Hooks/EventSingleView.php:

namespace MyVendor\MySeminar\Hooks;

use \OliverKlee\Seminars\Hooks\Interfaces\SeminarSingleView;
class EventSingleView implements SeminarSingleView
{

public function modifySingleView(\OliverKlee\Seminars\FrontEnd\DefaultController $controller): void
{
// deprecated
//   $data = $controller->pi_getRecord('tx_seminars_seminars', $controller->piVars['showUid']);
$precondition = $controller->getPrecondition();
$controller->setMarker('PRECONDITION', $precondition);
}

4. I registered both classes in the file ext_localconf.php:

$GLOBALS['TYPO3_CONF_VARS']['EXTCONF']['seminars'][\OliverKlee\Seminars\Hooks\Interfaces\SeminarSingleView::class][] = \MyVendor\MySeminar\Hooks\EventSingleView::class; $GLOBALS['TYPO3_CONF_VARS']['SYS']['Objects'][\OliverKlee\Seminars\Model\Event::class] = \MyVendor\MySeminar\Domain\Model\Event::class;

How can I get the value of $precondition and show it in the marker PRECONDITION?

I finally found out how to extend seminars with custom fields. This way it works:

my_seminar/ext_localconf.php

use MyVendor\MySeminar\Domain\Model\Event;

if (!defined('TYPO3_MODE')) die ('Access denied.');

$GLOBALS['TYPO3_CONF_VARS']['SYS']['Objects'][\OliverKlee\Seminars\OldModel\LegacyEvent::class] = [
    'className' => MyVendor\MySeminar\Domain\Model\Event::class
];

$GLOBALS['TYPO3_CONF_VARS']['EXTCONF']['seminars'][\OliverKlee\Seminars\Hooks\Interfaces\SeminarSingleView::class] = [
    'className' => MyVendor\MySeminar\Hooks\EventSingleView::class
];

my_seminar/Configuration/Extbase/Persistence/Classes.php

return [
    \MyVendor\MySeminar\Domain\Model\Event::class => [
        'tableName' => 'tx_seminars_seminars',
    ],
];

my_seminar/Classes/Domain/Model/Event.php

namespace MyVendor\MySeminar\Domain\Model;

class Event extends \OliverKlee\Seminars\OldModel\LegacyEvent
{
     /**
     * precondition
     *
     * @var string
     */
    protected $precondition = '';


     /**
     * Returns the precondition
     *
     * @return string $precondition
     */
    public function getPrecondition()
    {
         return $this->getTopicString('precondition');
    }

    /**
     * Sets the precondition
     *
     * @param string $precondition
     * @return void
     */
    public function setPrecondition($precondition)
    {
        $this->precondition = $precondition;
    }
}

my_seminar/Classes/Hooks/Event.php

namespace MyVendor\MySeminar\Hooks;
use OliverKlee\Seminars\Hooks\Interfaces\SeminarSingleView;
use OliverKlee\Seminars\Frontend\DefaultController;
class EventSingleView implements SeminarSingleView
{

    /**
     * Modifies the seminar details view.
     *
     * This function will be called for all types of seminars (single events, topics, and dates).
     *
     * @param DefaultController $controller the calling controller
     */

    public function modifySingleView(\OliverKlee\Seminars\FrontEnd\DefaultController $controller): void
    {
        $seminar = $controller->getSeminar();
        $precondition = $seminar->getPrecondition();
   
        if ($precondition != '') {
            $controller->setMarker('precondition', $precondition);
        } else {
            $controller->hideSubparts('precondition', 'field_wrapper');
        }
    }
}

Furthermore you have to adjust TCA/Overrides/tx_seminars_seminars.php and ext_tables.sql.