It is intentionally blocked from being a default, I have no idea why, but when you select it, it outright states it can't be done, so to get the feature you would need to request from Xerox officially to create a FER (Feature Enhancement Request). Which means you need to call and ask for it (we can't do it here) and there is no guarantee they will do so, it is just a request.
I will add, that I also just tried on the other platforms I have, which is Connectkey, VersaLink, FujiXerox, Discovery and none of them allow it either, which confirms it isn't a bug, it is definitely a "feature"