I am always forgetting things. This is one of them. There are two different key combinations with which you can start command line (cmd) when in WinPE (Windows PreInstallation Environment).
In normal Windows Setup, when you just boot installation media from DVD or USB flash disk, you can press Shift-F10 to open command line.
In the case of System Center Configuration Manager (shortly SCCM, configmgr or even config manager) and its WinPE OSD installation or image capture, the boot image has a different key for that. You must press F8.