Hi,
You can also create node by using the Rectangle class of Diagram.
[C#]
Syncfusion.Windows.Forms.Diagram.Rectangle nodeRectHost1 = new Syncfusion.Windows.Forms.Diagram.Rectangle(50, 100, 100, 50);
this.DiagramWebControl1.Model.AppendChild(nodeRectHost1);
Please refer the sample in the below link which illustrates the above.
http://websamples.syncfusion.com/samples/Diagram.Web/5.1.1.0/F65548_11/main.htmPlease try this and let me know if this helps.
Thanks,
Melba