Tracking codes are associated with line items, not transaction forms. This convention holds throughout the program. So, for example, you can set a default tracking code to appear every time a specific inventory item is used or a new one is created. But applying a code to everything on a form goes against the underlying concept of tracking codes.
Form Defaults will not work for what @novica wants. The first line item on a new form would carry the default tracking code. Others would not. Further, every form of the type involved would have the same tracking code on its first line item, which negates the purpose of tracking codes.