Default field validation- Activitiy

 Hi,

I am trying to create a task based on the following scenario

1. When a task is created without an account name the status field must be 'assigned'

2. When a task is created with an account name the status field must be 'Completed'. The completed date must be populated automatically.

I am trying to achieve this by using the following condition in the default value of the status field

IIf((FieldValue('<AccountId>') IS NOT NULL), 'Completed', 'Assigned')

However i am able to achive both the scenario but the activity completed date is not getting populated and the activity still remains in the open activities section with the status completed.