<?php

# placed in EXT:site/Configuration/SiteConfiguration/Overrides/sites.php

defined('TYPO3_MODE') || die('Access denied.');

call_user_func(
    function ($table) {
       
        $GLOBALS['SiteConfiguration'][$table]['columns']['googleTagManager'] = [
            'label' => 'Google Tag Manager',
            'config' => [
                'type' => 'input',
                'placeholder' => 'GTM-123456',
            ],
        ];

        $GLOBALS['SiteConfiguration'][$table]['types']['0']['showitem'] .= ',--div--;Custom,googleTagManager';
    },
    'site'
);