ext_tables.php

<?php
/******************************************************************************
 * Linkbox Plugin
 ******************************************************************************/

$_EXTKEY = 'mfc_linkbox';

\TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addPlugin(
    array(
        'LLL:EXT:mfc_linkbox/Resources/Private/Language/locallang_db.xml:tt_content.list_type_pi1',
        $_EXTKEY . '_pi1'
    ),
    'list_type'
);

\TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addStaticFile($_EXTKEY, 'Configuration/TypoScript/', 'Linkbox');

$TCA['tt_content']['types']['list']['subtypes_addlist'][$_EXTKEY . '_pi1'] = 'pi_flexform';

\TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addPiFlexFormValue(
    $_EXTKEY . '_pi1',
    'FILE:EXT:' . $_EXTKEY . '/flexform.xml'
);

// remove some fields from the tt_content content element
$TCA['tt_content']['types']['list']['subtypes_excludelist'][$_EXTKEY . '_pi1'] = 'layout,select_key,pages,recursive';