test.cs

public IActionResult Create(FormData data) {

  SignData signData = new SignData();

  byte[] document;

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

    using (TXTextControl.DocumentServer.MailMerge mm = new TXTextControl.DocumentServer.MailMerge()) {
      mm.TextComponent = tx;
      mm.FormFieldMergeType = TXTextControl.DocumentServer.FormFieldMergeType.Preselect;
      mm.RemoveEmptyFields = false;

      // pre-select form fields with known data and merge
      // data into merge fields
      mm.MergeObject(data);
    }

    tx.Save(out document, TXTextControl.BinaryStreamType.InternalUnicodeFormat);

  }

  signData.document = document;
  signData.formData = data;

  return View(signData);
}