.WithSignatureStyle(SignatureStyle.HAND_DRAWN)

I found it is actually .WithStyle
documentBuilder.WithSignature(SignatureBuilder.SignatureFor(signer.Email)

                                       .OnPage(int.Parse(signatureDetails[i].PageNumber))                                        .AtPosition(int.Parse(signatureDetails[i].XPosition), int.Parse(signatureDetails[i].YPosition))                                        .WithStyle(SignatureStyle.HAND_DRAWN));