Error message ? I suspect it won't works as the value requires to be in the format 1;#Domain\User. That means you have to know the ID in advance, and thus, probably a custom feature receiver will be required (or EventReceiver with listadded event, depending on your needs) – Steve BJan 18 '12 at 16:16
@SteveB: I'd recommend adding that as an answer – Stuart PeggAug 14 '12 at 20:14
1;#Domain\User. That means you have to know the ID in advance, and thus, probably a custom feature receiver will be required (or EventReceiver with listadded event, depending on your needs) – Steve B Jan 18 '12 at 16:16