Visual Studio Workflow 2013 - Single Task action

Question

I use the SingleTask activity in my Workflow to create Tasks for the User. But I need to give the User more information than just Body, Title, DueDate and AssignTo. So I created a Content Type which inherits from Workflow Task (SP2013) and it has a Site Column called TestField. So when the Single Task is created, I want to programatically add the TestField-field as soon as this Task is created. But I also need to wait for completion of the Task to continue with the Workflow. The TestField field value is needed for the Task to get finished, so I can't add it after the completion of the task. I need to give this Value to the Task after/while creation. Is this possible? Any idea? Can anybody help me?