test.cs

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

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

  using (TXTextControl.DocumentServer.MailMerge mailMerge = new TXTextControl.DocumentServer.MailMerge()) {
    mailMerge.TextComponent = tx;
    mailMerge.SearchPath = Application.StartupPath;
    var jsonData = System.IO.File.ReadAllText("data.json");

    mailMerge.MergeJsonData(jsonData);
  }
}