OK, I changed that into:
scratchpads = [
     NS "calendar" "urxvt -name calendar -e task calendar" (title =? "calendar")
     (customFloating $ W.RationalRect 0 (1/10) (4/10) (3/10))
 ]

Now it compiles without an error. But when I type <Mod-d>,
a window pops up and disappears. I changed the »task calendar«
command into »htop«. Then a terminal with htop is opened. But
it's not floated.