Workflow Set Delegated User

I am attempting to create a workflow that sets the delegated user ID to a specific user each time an activity is created.  I do not know how to get the syntax correct to set the Delegated ID to a specific user??  Here is what I attempted . . .[<DelegatedById>]=UserValue(NATE.JOHNSON).  What/Should I be using the UserValue expression?

My workflow setup is . . .

Record Type = Activity

Trigger Event = When a New Record Is Saved

Workflow Rule Condition = [<DelegatedById>] IS NULL (We are not exposing this to end users)

Action Item 1 = Wait 0

Action Item 2 = Expression above.

Thanks,

Nate