Put
Admins;Role Administrators
Roles have both a role name and a display name, the Administrators role name is Admins, though I know that is a little confusing it can't be changed for historical reasons. The display name of roles can be edited but the role name cannot.
Note that onlyAvailableForRoles means this property is only available on users who are members of those roles, ie it won't be a property on users who are not in those roles.
Hope that helps,
Joe