test.cs

private void button1_Click(object sender, EventArgs e)
{
   textControl1.Load("App_Data/invoice.tx", TXTextControl.StreamType.InternalUnicodeFormat);

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

   using (var mailMerge = new MailMerge { TextComponent = textControl1 })
   {
       mailMerge.MergeJsonData(jsonData, true);
   }
}