test.cs

[CustomActionFilter]
[HttpPost]
public IActionResult HandleSignature([FromBody] SignatureData data) {

  byte[] bPDF;

  // create temporary ServerTextControl
  using (TXTextControl.ServerTextControl tx = new TXTextControl.ServerTextControl()) {
    tx.Create();

    // load the document
    tx.Load(Convert.FromBase64String(data.SignedDocument.Document), TXTextControl.BinaryStreamType.InternalUnicodeFormat);

    //FlattenFormFields(tx);

    X509Certificate2 cert = new X509Certificate2("App_Data/textcontrolself.pfx", "123");

    var signatureFields = new List<DigitalSignature>();

    foreach (SignatureBox box in data.SignatureBoxes) {
      signatureFields.Add(new DigitalSignature(cert, null, box.Name));
    }

    TXTextControl.SaveSettings saveSettings = new TXTextControl.SaveSettings() {
      CreatorApplication = "Your Application",
      SignatureFields = signatureFields.ToArray()
    };

    // store the PDF in the database or send it to the client
    tx.Save(out bPDF, TXTextControl.BinaryStreamType.AdobePDFA, saveSettings);

    // alternatively, save the PDF to a file
    tx.Save("App_Data/signed.pdf", TXTextControl.StreamType.AdobePDFA, saveSettings);
  }

  // return any value to the client
  return Ok();
}