Nextcloud desktop sync client does not integrate with explorer (file manager) in windows 8.1

On https://docs.nextcloud.com/desktop/3.3/navigating.html i read:
“The Nextcloud desktop sync client integrates with your file manager. Finder on macOS and Explorer on Windows.”
But on my german Windows 8.1 it does not. There is no Nextcloud option in the context menu (right mouse button) with Windows Client version 3.3.5.