DigitalSignatureDetails

public DigitalSignatureDetails DigitalSignatureDetails { get; set; }

Document doc = new Document(MyDir + "Document.docx");

CertificateHolder certificateHolder = CertificateHolder.Create(MyDir + "morzal.pfx", "aw");
OoxmlSaveOptions saveOptions = new OoxmlSaveOptions();
saveOptions.DigitalSignatureDetails = new DigitalSignatureDetails(
    certificateHolder,
    new SignOptions() { Comments = "Some comments", SignTime = DateTime.Now });

doc.Save(ArtifactsDir + "OoxmlSaveOptions.DigitalSignature.docx", saveOptions);