How do I create an invisible digital signatue field?

I want to digitally sign a document but create an invisible signature. How can I do that?

You can follow the sample code here to create a signature but create a field with 0 width and height, which will make it invisible:

SignatureWidget widgetAnnot = SignatureWidget.Create(doc, new Rect(0, 0, 0, 0), certification_sig_field);