I've got it... In ext_tables:
$myPid =
intval(unserialize($GLOBALS['TYPO3_CONF_VARS']['EXT']['extConf']['my_ext'])['myPid']);
\TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addPageTSConfig('
[PIDinRootline = ' . $myPid . ']
...
[]
');
Kind regards, Jan