[TYPO3-mvc] getConfiguration CONFIGURATION_TYPE_SETTINGS form a namespaced v6 extension

[Jan Kiesewetter]

If i try to get the settings of my extension from inside a singleton i always get NULL via:

$this->configurationManager->getConfiguration(\TYPO3\CMS\Extbase\Configuration\ConfigurationManagerInterface::CONFIGURATION_TYPE_SETTINGS, 'extensionName', 'pluginName');

What do i have to set for extensionName for a namespaced extension with verdor prefix?
I tried "Vendor.Extensionkey" (like configurePlugin), "extensionkey"