Gets the first open manifest available. If there are no open manifests available, it creates one and returns it.
UserID - If the manifest has to be created, this is the UserID to create against.
AutoImport - Optional, if omitted defaults to 'false'. This sets the AutoImport flag on the manifest, which allows invalid consignments to be added for correction later. It also makes the manifest show up in 'Auto Imported Manifests' instead of 'Open Manifests'.
The same data structure as the List Manifests method.