speedbase/Configuration/TCA/Overrides/tt_content.php

<?php
defined('TYPO3') || die();

use TYPO3\CMS\Core\Utility\ExtensionManagementUtility;

ExtensionManagementUtility::addTcaSelectItem(
'tt_content',
'CType',
[
    'LLL:EXT:speedbase/Resources/Private/Language/locallang.xlf:speedbase_ctype_title',
    'speedbase',
    'content-text',
],
'textmedia',
'after'
);

$GLOBALS['TCA']['tt_content']['types']['speedbase'] = [
    'showitem' => '
        --palette--;LLL:EXT:frontend/Resources/Private/Language/locallang_ttc.xlf:palette.general;general,
        --palette--;LLL:EXT:frontend/Resources/Private/Language/locallang_ttc.xlf:palette.headers;modified_header,
        --div--;LLL:EXT:speedbase/Resources/Private/Language/locallang_ttc.xlf:tabs.custom_fields,
            tx_speedbase_chatbotterms, tx_speedbase_chatbottermscategory, tx_speedbase_chatbottermstype, tx_speedbase_chatbottermsurl,
        --div--;LLL:EXT:frontend/Resources/Private/Language/locallang_ttc.xlf:tabs.appearance,
            --palette--;LLL:EXT:frontend/Resources/Private/Language/locallang_ttc.xlf:palette.frames;frames,
        --div--;LLL:EXT:core/Resources/Private/Language/Form/locallang_tabs.xlf:language,
            --palette--;;language,
        --div--;LLL:EXT:core/Resources/Private/Language/Form/locallang_tabs.xlf:access,
            --palette--;LLL:EXT:frontend/Resources/Private/Language/locallang_ttc.xlf:palette.visibility;visibility,
            --palette--;LLL:EXT:frontend/Resources/Private/Language/locallang_ttc.xlf:palette.access;access,
    ',
];

// Add custom fields to the tt_content TCA
ExtensionManagementUtility::addTCAcolumns('tt_content', [
    'tx_speedbase_chatbotterms' => [
        'label' => 'LLL:EXT:speedbase/Resources/Private/Language/locallang_db.xlf:tt_content.chatbotterms',
        'config' => [
            'type' => 'text',
            'size' => 2000,
            'eval' => 'trim',
        ],
    ],
    'tx_speedbase_chatbottermscategory' => [
        'label' => 'LLL:EXT:speedbase/Resources/Private/Language/locallang_db.xlf:tt_content.chatbottermscategory',
        'config' => [
            'type' => 'select',
            'renderType' => 'selectSingle',
            'default' => '',
            'items' => [
                ['Default', '--div--'],
                ['--leer--', null ],
                ['DB values', '--div--'],
            ],
            'foreign_table' => 'tx_begriffmgt_domain_model_category',
        ],
    ],
    'tx_speedbase_chatbottermstype' => [
        'label' => 'LLL:EXT:speedbase/Resources/Private/Language/locallang_db.xlf:tt_content.chatbottermstype',
        'config' => [
            'type' => 'select',
            'renderType' => 'selectSingle',
#            'default' => '',
            'items' => [
                ['Default', '--div--'],
                ['--leer--', null ],
                ['DB values', '--div--'],
            ],
            'foreign_table' => 'tx_begriffmgt_domain_model_type',
        ],
    ],
    'tx_speedbase_chatbottermsurl' => [
        'label' => 'LLL:EXT:speedbase/Resources/Private/Language/locallang_db.xlf:tt_content.chatbottermsurl',
        'config' => [
            'type' => 'input',
            'size' => 50,
            'eval' => 'trim',
        ],
    ],
]);

// Create a new 'modified_header' palette with only the 'header' field
    $GLOBALS['TCA']['tt_content']['palettes']['modified_header'] = [
        'showitem' => 'header'
    ];