test.cs

TXTextControl.LoadSettings ls = new TXTextControl.LoadSettings() {
  ApplicationFieldFormat = TXTextControl.ApplicationFieldFormat.MSWord };

using (TXTextControl.ServerTextControl serverTextControl =
       new TXTextControl.ServerTextControl()) {
  serverTextControl.Create();
  serverTextControl.Load("template.docx", TXTextControl.StreamType.WordprocessingML, ls);

  using (MailMerge mailMerge = new MailMerge()) {
    mailMerge.TextComponent = serverTextControl;
  }
}