How can I programmatically create replies to annotations?

A: You can create replies to markup annotations programmatically in your application by using the following code (C++):

PDFDoc doc("input.pdf"); // input doc
Page pg = doc.GetPage(x); // page the annotation is on
Annot parent = pg.GetAnnot(y); // The annotation that the user will reply to

Rect rect = parent.GetRect();

double left = rect.GetX1();
double top = rect.GetY2();

Point p(left, top);
PDF::Annots::Text reply = PDF::Annots::Text::Create(doc, p);
reply.SetTitle("AUTHOR"); // this function is used to set the author/user of the reply
reply.GetSDFObj().Put("IRT", parent.GetSDFObj());
Rect popuprect;
popuprect.Set(left + 20, top + 20, left + 90, top + 90);
PDF::Annots::Popup popup = PDF::Annots::Popup::Create(doc, popuprect);
popup.SetContents("Comment that the person made"); // the users comment goes here