mrm_be/Configuration/TCA/Overrides/custom_components/infobox.php

<?php

\TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addTcaSelectItem(
   'tt_content',
   'CType',
    [
        'LLL:EXT:mrm_be/Resources/Private/Language/locallang_ttc.xlf:mrm_be_cm_infobox.title',
        'mrm_be_cm_infobox',
        'mrm_be_cm_infobox_icon',
    ]
);

$GLOBALS['TCA']['tt_content']['columns']['pi_flexform']['config']['ds'][',mrm_be_cm_infobox'] = 'FILE:EXT:mrm_be/Configuration/FlexForms/infobox.xml';
$GLOBALS['TCA']['tt_content']['types']['mrm_be_cm_infobox'] = [
    'showitem' => 'CType;;;1-1-1,pi_flexform;LLL:EXT:mrm_be/Resources/Private/Language/locallang_ttc.xlf:mrm_be_cm_infobox.title,',
];