How do I create a digital signature annotation and set its appearance?

There is now an API on the new Annots.SignatureWidget class.

SignatureWidhet.CreateSignatureAppearance(Image)
https://www.pdftron.com/api/PDFTronSDK/dotnet/pdftron.PDF.Annots.SignatureWidget.html#pdftron_PDF_Annots_SignatureWidget_CreateSignatureAppearance_pdftron_PDF_Image_