test.cs

using (TXTextControl.DocumentServer.MailMerge mm = new TXTextControl.DocumentServer.MailMerge()) {
  mm.TextComponent = textControl1;

  mm.IncludeTextMerging += Mm_IncludeTextMerging;
  mm.MergeJsonData(sJsonData);
}

private void Mm_IncludeTextMerging(object sender, TXTextControl.DocumentServer.MailMerge.IncludeTextMergingEventArgs e) {

  if (e.IncludeTextField.Filename == "subtemplate.docx") {

    byte[] data;

    using (TXTextControl.ServerTextControl tx = new TXTextControl.ServerTextControl()) {
      tx.Create();
      tx.Load("<span style=\"color: red\">This</span> is the content that should be <strong>merged into the IncludeText field</strong>.",
        TXTextControl.StringStreamType.HTMLFormat);

      tx.Save(out data, TXTextControl.BinaryStreamType.InternalUnicodeFormat);
    }

    e.IncludeTextDocument = data;

  }
}