<?php

\TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addTcaSelectItem(
    'tt_content',
    'CType',
     [
       'LLL:EXT:mrm_be/Resources/Private/Language/locallang_ttc.xlf:mrm_be_cm_maintenance.title',
       'mrm_be_cm_maintenance',
       'mrm_be_cm_maintenance_icon',
     ]
   );
   
   $GLOBALS['TCA']['tt_content']['columns']['pi_flexform']['config']['ds'][',mrm_be_cm_maintenance'] = 'FILE:EXT:mrm_be/Configuration/FlexForms/maintenance.xml';
   $GLOBALS['TCA']['tt_content']['types']['mrm_be_cm_maintenance'] = [
     'showitem' => 'CType;;;1-1-1,pi_flexform;LLL:EXT:mrm_be/Resources/Private/Language/locallang_ttc.xlf:mrm_be_cm_maintenance.title,',
   ];