cannot assign task by confluence api

[chenkai]

I create a page by confluence api with content like  the follow code, the page has been created success, but I can't see the task in my task list, and after reedit this page by  the  web , I can see that task in my task list. 

POST /rest/api/content : 
{ 
  "type":"page", 
  "title":"Another planning specs with username", 
  "body":{ 
           "storage":{
              "value": "<ac:task-list> <ac:task> <ac:task-status>incomplete</ac:task-status> <ac:task-body> <ac:link><ri:user ri:username='admin' /></ac:link> do something </ac:task-body> </ac:task> </ac:task-list>", 
              "representation":"storage"
           }
  },
  "space":{"key":"TST"} 
}