Add fields to ext:news v11 flexform with TYPO3 v12

AfterFlexFormDataStructureParsedEventListener.php

<?php
declare(strict_types=1);

namespace Vendor\Extension\EventListener;

use TYPO3\CMS\Core\Configuration\Event\AfterFlexFormDataStructureParsedEvent;

class AfterFlexFormDataStructureParsedEventListener
{

    public function __invoke(AfterFlexFormDataStructureParsedEvent $event)
    {
        $dataStructure = $event->getDataStructure();
        $identifier = $event->getIdentifier();

        // key could be e.g. "*,news_newssearchform"
        if ($identifier['type'] === 'tca' && $identifier['tableName'] === 'tt_content' && str_starts_with($identifier['dataStructureKey'], '*,news_')) {

            // instead of "inline", you could also use GeneralUtility::xml2array(file_get_contents(ExtensionManagementUtility::extPath('extension') . 'Configuration/FlexForms/Custom.xml'));
            $dataStructure['sheets']['additional'] = [
                'ROOT' => [
                    'sheetTitle' => 'My custom',
                    'type' => 'array',
                    'el' => [
                        'settings.fo' => [
                            'label' => 'A label',
                            'config' => [
                                'type' => 'check',
                                'default' => 0,
                            ],
                        ],
                    ],
                ],
            ];
        }
        $event->setDataStructure($dataStructure);
    }
}

Services.yaml

services:
  _defaults:
    autowire: true
    autoconfigure: true
    public: false

  Vendor\Extension\:
    resource: '../Classes/*'

  Vendor\Extension\EventListener\AfterFlexFormDataStructureParsedEventListener:
    tags:
      - name: event.listener
        identifier: 'extension-flexformhook'
        event: TYPO3\CMS\Core\Configuration\Event\AfterFlexFormDataStructureParsedEvent