Makes the annotation or XObject target the element's last content item (an object reference).
public PdfObjectReference AddObjectReference(CosReference target, PdfPage? page)
The indirect reference to the annotation dictionary or XObject stream.
The page the object is on, written as the OBJR's Pg; null to leave it out.
ISO 32000-2 §14.7.5.3 (Table 358), §14.7.5.4 (Table 359): the object gets a StructParent key, and the parent tree maps that
key to this element.