Xfce Keyboard Layout change Ctrl+Space

I've been googling this for almost a week to no avail.  When I open the Keyboard settings pane, the options (ones that have "Space" in them) are limited to only Alt+Space and Super+Space – there is no Ctrl+Space. I've found a couple of answers, none of which seem to work. The first one is to install the xfce4-xkb-plugin (which I already had installed), then set "use system default" flag in the Keyboard settings pane, after which use plugin Properties to set the languages and shortcuts.  It doesn't work – the Properties pane has only options on how the pane item looks...that's it. Number two is this line: -option grp:ctrl_space_toggle,grp_led:scroll en,ru It doesn't work either.  If I put this in my .Xmodmap file, there is an error and the file isn't parsed anymore. I gave up on searching, but I really need this, as it's a long time reflex.  Please help me set Ctrl+Space as a shortcut to change layout.  (I use Arch Linux.)