Creating a custom scheduler task 

<?php

declare(strict_types=1);

namespace MyVendor\MyExtension\Task;

use MyVendor\MyExtension\BusinessLogic;
use TYPO3\CMS\Core\Utility\GeneralUtility;
use TYPO3\CMS\Scheduler\Task\AbstractTask;

final class MyTask extends AbstractTask
{
    /**
     * MUST be implemented by all tasks
     */
    public function execute(): bool
    {
        # Dependency injection cannot be used in scheduler tasks
        $businessLogic = GeneralUtility::makeInstance(BusinessLogic::class);
        return $businessLogic->run('arg1', 'arg2', '…');
    }

    public function getAdditionalInformation()
    {
        $this->getLanguageService()->sL('LLL:EXT:my_extension/Resources/Private/Language/locallang.xlf:myTaskInformation');
    }
}

<?php

declare(strict_types=1);

use MyVendor\MyExtension\Task\MyTask;
use TYPO3\CMS\Core\Utility\ExtensionManagementUtility;

defined('TYPO3') or die();

if (isset($GLOBALS['TCA']['tx_scheduler_task'])) {
    ExtensionManagementUtility::addRecordType(
        [
            'label' => 'My Custom Task',
            'description' => 'Description of what this task does',
            'value' => MyTask::class,
            'icon' => 'my-custom-icon',
            'iconOverlay' => 'content-clock',
            'group' => 'my_extension',
        ],
        $GLOBALS['TCA']['tx_scheduler_task']['types']['0']['showitem'],
        [],
        '',
        'tx_scheduler_task'
    );
}

<?php

declare(strict_types=1);

use TYPO3\CMS\Core\Utility\ExtensionManagementUtility;
use MyVendor\MyExtension\Task\MyTask;

defined('TYPO3') or die();

if (isset($GLOBALS['TCA']['tx_scheduler_task'])) {
    // Add custom fields to the tx_scheduler_task table
    ExtensionManagementUtility::addTCAcolumns(
        'tx_scheduler_task',
        [
            'my_extension_field' => [
                'label' => 'LLL:EXT:my_extension/Resources/Private/Language/locallang.xlf:field.label',
                'config' => [
                    'type' => 'input',
                    'size' => 30,
                    'required' => true,
                    'eval' => 'trim',
                    'placeholder' => 'Enter value here...',
                ],
            ],
            'my_extension_email_list' => [
                'label' => 'LLL:EXT:my_extension/Resources/Private/Language/locallang.xlf:emailList.label',
                'config' => [
                    'type' => 'text',
                    'rows' => 3,
                    'required' => true,
                    'placeholder' => 'admin@example.com',
                ],
            ],
        ]
    );

    // Register the task type
    ExtensionManagementUtility::addRecordType(
        [
            'label' => 'Some title or LLL:EXT reference',
            'description' => 'Some description or LLL:EXT reference',
            'value' => MyTask::class,
            'icon' => 'mimetypes-x-tx_scheduler_task_group',
            'iconOverlay' => 'content-clock',
            'group' => 'my_extension',
        ],
        '
            --div--;core.form.tabs:general,
                tasktype,
                task_group,
                description,
                my_extension_field,
                my_extension_email_list,
            --div--;core.form.tabs:timing,
                --palette--;;execution,
            --div--;core.form.tabs:access,
                disable,
            --div--;core.form.tabs:extended,',
        [],
        '',
        'tx_scheduler_task'
    );
}

<?php

declare(strict_types=1);

namespace MyVendor\MyExtension\Task;

use MyVendor\MyExtension\BusinessLogic;
use TYPO3\CMS\Core\Messaging\FlashMessage;
use TYPO3\CMS\Core\Messaging\FlashMessageService;
use TYPO3\CMS\Core\Type\ContextualFeedbackSeverity;
use TYPO3\CMS\Core\Utility\GeneralUtility;
use TYPO3\CMS\Scheduler\Task\AbstractTask;

final class MyTask extends AbstractTask
{
    protected string $myField = '';
    protected string $emailList = '';

    public function execute(): bool
    {
        # Dependency injection cannot be used in scheduler tasks
        $businessLogic = GeneralUtility::makeInstance(BusinessLogic::class);
        return $businessLogic->run($this->myField, $this->emailList, '…');
    }

    /**
     * Set field values from associative array.
     *
     * @param array $parameters Values from TCA fields
     */
    public function setTaskParameters(array $parameters): void
    {
        $this->myField = $parameters['my_extension_field'] ?? '';
        $this->emailList = $parameters['my_extension_email_list'] ?? '';
    }

    /**
     * Validate task parameters.
     * Only implement this method for validation that cannot be handled by FormEngine.
     * Basic validation like 'required' should be done via TCA 'eval' configuration.
     */
    public function validateTaskParameters(array $parameters): bool
    {
        $isValid = true;

        // Example: Custom email validation (beyond basic 'required' check)
        $emailList = $parameters['my_extension_email_list'] ?? '';
        if (!empty($emailList)) {
            $emails = GeneralUtility::trimExplode(',', $emailList, true);
            foreach ($emails as $email) {
                if (!GeneralUtility::validEmail($email)) {
                    GeneralUtility::makeInstance(FlashMessageService::class)
                        ->getMessageQueueByIdentifier()
                        ->addMessage(
                            GeneralUtility::makeInstance(
                                FlashMessage::class,
                                'Invalid email address: ' . $email,
                                '',
                                ContextualFeedbackSeverity::ERROR
                            )
                        );
                    $isValid = false;
                }
            }
        }

        return $isValid;
    }
    public function getAdditionalInformation(): string
    {
        return sprintf(
            'Field: %s, Emails: %s',
            $this->myField,
            $this->emailList
        );
    }
}