Hello Dennis,
At the time of creation and fetching additional information using custom keys, you need to provide the permission of Storage service methods. Because additional data in User creation is going to save in No-SQL Database. So you need to provide the access of InsertJSONDocument method and fetch method of Storage service to find the additional information.
Please add above information from Key Management under App Manager (App Manager -> Custom key -> Manage Access) section of AppHQ Management Console and let us know if it helps.
Regards.
Himanshu Sharma