QMS API How can I add a tasktriggering to a task?

[#1]

I'm trying to creat a task trggerred by external event and I have already created the task. However when I tried to add a trigger to the task, it didn't work. It didn't fail or have any error message, just the task triggering was always none. Can someone tell me why? Is it a server side issue? Here is my code to add the trigger:

   ExternalEventTrigger st = new ExternalEventTrigger();

   st.Type = TaskTriggerType.ExternalEventTrigger;

   st.Enabled = true;

   st.ID = Guid.NewGuid();

   QMSAPIService.Trigger[] List_Trigger = new QMSAPIService.Trigger[1];

   //add triggering

   documentTask.Triggering = new DocumentTask.TaskTriggering();

   documentTask.Triggering.TaskDependencies = new List<TaskInfo>().ToArray();

   documentTask.Scope = DocumentTaskScope.General;