I don't think the Warforged from "Wayfinders Guide to Eberron" can be done by custom. Those things needs some serious formula to work the armor.
@calcu10n I did the same thing, checking a supported subclass vs the custom page subclass, and saw the same results. I think it is either something FA never put in for custom subclasses, or it is a bug in the beta. Either way, the only way I can think of to add that information is to use the Start pages Additional Details (below the Character Details box) and manually type it in. Information in the Additional Details box will be added to the end of the Character Details.