test.cs

// get the SignatureField by Name
TXTextControl.SignatureField signatureField = 
    textControl1.SignatureFields.GetItem("txsign_1");

// set the image
signatureField.Image = new TXTextControl.SignatureImage("signature.svg", 1);

// create a new digital signature
X509Certificate2 cert = new X509Certificate2("mycert.pfx", "123");
TXTextControl.DigitalSignature signature = 
    new TXTextControl.DigitalSignature(cert, null, "txsign_1");

// add the signatures
TXTextControl.SaveSettings saveSettings = new TXTextControl.SaveSettings() {
    SignatureFields = new TXTextControl.DigitalSignature[] { signature }
};

// export the document
textControl1.Save("results.pdf", TXTextControl.StreamType.AdobePDF, saveSettings);