test.cs

[HttpPost]
[Route("CustomSign")]
public string CustomSign([FromBody] SignatureData signatureData)
{
  byte[] pdfBytes;

  using (var tx = new ServerTextControl()) {
    tx.Create();
    tx.Load("App_Data/template.tx", StreamType.InternalUnicodeFormat);

    // load the signature image into the signature fields
    foreach (var box in signatureData.SignatureBoxes) {
      foreach (SignatureField field in tx.SignatureFields) {
        if (field.Name == box.Name) {
          var stamp = Convert.FromBase64String(signatureData.SignatureImage);

          using (var ms = new MemoryStream(stamp, 0, stamp.Length, false, true)) {
            field.Image = new SignatureImage(ms);
          }
        }
      }
    }

    // fill the form fields with the data
    foreach (TXTextControl.FormField formField in tx.FormFields) {
      foreach (var dataField in signatureData.FormFields) {
        if (dataField.Name == formField.Name) {
          formField.Text = dataField.Value;
          break;
        }
      }
    }

    FlattenFormFields(tx);
    tx.Save(out pdfBytes, BinaryStreamType.AdobePDF);
  }

  return Convert.ToBase64String(pdfBytes);
}