AddPointAnnotationInPDF.cs

// For complete examples and data files, please go to https://github.com/groupdocs-annotation/GroupDocs.Annotation-for-.NET
using (Annotator annotator = new Annotator("input.pdf"))
{
    PointAnnotation point = new PointAnnotation
    {
        Box = new Rectangle(100, 100, 0, 0),
        CreatedOn = DateTime.Now,
        PageNumber = 0
    };
    annotator.Add(point);
    annotator.Save("result.pdf"));
}