Where did you place this macro? By Default, it isn't available for you to set a default value that I've seen. The article Roman provided didn't provide any detail as to where this might be set on a more global level, so I'm curious now.
From what I'm seeing you have to place that macro in the PublishTo field every time you create a new document. This requires user input and interaction. The solution I was providing allows it to be done automatically for any page types you specify.