test.cs

using (TXTextControl.ServerTextControl tx = new TXTextControl.ServerTextControl()) {

  tx.Create();
  tx.Load("patient_history.tx", TXTextControl.StreamType.InternalUnicodeFormat);

  using (TXTextControl.DocumentServer.MailMerge mailMerge = new TXTextControl.DocumentServer.MailMerge()) {
    mailMerge.TextComponent = tx;

    mailMerge.FormFieldMergeType = TXTextControl.DocumentServer.FormFieldMergeType.Preselect;

    string jsonData = File.ReadAllText("data.json");
    mailMerge.MergeJsonData(jsonData);
  }

}