test.cs

// enable MS Word merge fields
TXTextControl.LoadSettings ls = new TXTextControl.LoadSettings() {
  ApplicationFieldFormat = TXTextControl.ApplicationFieldFormat.MSWord };

// load JSON data
string jsonData = System.IO.File.ReadAllText("data.json");

// create a temporary ServerTextControl
using (TXTextControl.ServerTextControl serverTextControl = 
       new TXTextControl.ServerTextControl()) {
  serverTextControl.Create();
  
  // load the template
  serverTextControl.Load("template.docx", TXTextControl.StreamType.WordprocessingML, ls);

  // create the mail merge engine
  using (MailMerge mailMerge = new MailMerge()) {
    // connect to ServerTextControl
    mailMerge.TextComponent = serverTextControl;
    // merge data into template
    mailMerge.MergeJsonData(jsonData);
  }
}