To offer the list of message boxes in the node, use the TypeConvert ClientJobStepMsgBoxListTypeConverter from the namespace eBiss.Api.Workflow (eBiss.Api.dll). E.g. in the following for the property MessageBoxName:

public class RedFromHdSample : ClientJobStepBase
{
        [TypeConverter(typeof(ClientJobStepMsgBoxListTypeConverter)]
        public string MessageBoxName { get; set; }
}