You will find it at Scene Manager > Output > Extra Sampling. However, I discovered last night that I missed a change in the SDK, so it doesn't currently work from the plugin.
Sorry, I somehow glossed over the fact that you were specifically referring to the Standalone plugin (by the way, there are specific forums for questions pertaining just to the Standalone or Render Suite versions). I'll have to ask whether this feature is supposed to be supported in this case (i.e. production engine, but standalone plugin).