setSceneGraphBackend() can't be used in Qt6?

In order to use QML with a Vulkan Backend with external Vulkan Devices, I used Qt 6.0.0 which allowed me to force QML to run with a Vulkan environment with the function 

void QQuickWindow::setSceneGraphBackend(QSGRendererInterface::GraphicsApi api)

But since the beta update, all that remains is the Qt 5.8 function 

void QQuickWindow::setSceneGraphBackend(const QString &backend)

 How can I use the 

QSGRendererInterface::Vulkan

or

QSGRendererInterface::VulkanRhi

enum?