using CadImage cadImage = (CadImage)Image.Load(fileName);

CadText text = new CadText();
text.DefaultValue = "Watermark text";
text.TextHeight = 40;
text.FirstAlignment = new Cad3DPoint(300, 40);
text.LayerName = "0";
cadImage.BlockEntities["*Model_Space"].AddEntity(text);

CadMText watermark = new CadMText();
watermark.Text = "Watermark message";
watermark.InitialTextHeight = 40;
watermark.InsertionPoint = new Cad3DPoint(300, 40);
watermark.LayerName = "0";
cadImage.BlockEntities["*Model_Space"].AddEntity(watermark);

cadImage.UpdateSize();