test.cs

public IActionResult Index()
{
    using (TXTextControl.ServerTextControl tx = new TXTextControl.ServerTextControl())
    {
        tx.Create();
        tx.Load("App_Data/gpay_application.tx", TXTextControl.StreamType.InternalUnicodeFormat);

        var jsonData = System.IO.File.ReadAllText("App_Data/data.json");

        using (MailMerge mm = new MailMerge())
        {
          mm.TextComponent = tx;
          mm.FormFieldMergeType = FormFieldMergeType.Preselect;
          mm.MergeJsonData(jsonData);
        }

        byte[] data;
        tx.Save(out data, TXTextControl.BinaryStreamType.InternalUnicodeFormat);

        ViewBag.Document = Convert.ToBase64String(data);
    }

    return View();
}