You'll need to create a new "Launcher" from the desktop and make sure "Type" is set to "Application":
Name it "Suspend" and for "Command" use: "dbus-send --system --print-reply --dest="org.freedesktop.UPower" /org/freedesktop/UPower org.freedesktop.UPower.Suspend"
Download this handy suspend icon image and then use it to set the icon for the launcher by clicking on the icon button:
