How to set default window manager for auto login?

I have Tumbleweed with the MATE desktop and autologin enabled, and also installed the Openbox WM. Now how could I autologin into Openbox instead of MATE? In YaST's /etc/sysconfig editor, in Desktop > Window manager > DEFAULT_WM, the possible values are: default, gnome, plasma5, lxde, xfce, twm, icewm, enlightenment or 'any value'. So openbox is not among the options, and if I enter 'openbox', it has no effect. I also tried 'update-alternatives --config default-xsession.desktop' as suggested in the sysconfig editor, but then I can only opt for mate.desktop or icewm-session.desktop.