test.cs

private void Mm_FieldMerged(
  object sender, 
  TXTextControl.DocumentServer.MailMerge.FieldMergedEventArgs e) {

  // check if the current field is a merge field
  if (e.MailMergeFieldAdapter.TypeName == "MERGEFIELD") {
    MergeField field = (MergeField)e.MailMergeFieldAdapter;

    // the DataRow returns the object of the actual data source
    if (e.DataRow[field.Name].GetType() == typeof(Hyperlink)) {

      // cast the object to a Hyperlink
      Hyperlink hyperlink = (Hyperlink)e.DataRow[field.Name];

      // use a temporary ServerTextControl to generate HypertextLink
      using (TXTextControl.ServerTextControl tx = new TXTextControl.ServerTextControl()) {
        tx.Create();

        // the load the field to apply the formatting
        tx.Load(e.MergedField, TXTextControl.BinaryStreamType.InternalUnicodeFormat);

        // remove the field
        tx.ApplicationFields.Clear(true);

        // remove the text, but keep formatting
        tx.SelectAll();
        tx.Selection.Text = "";

        // create and insert a new hypertext link based on the data row object
        TXTextControl.HypertextLink hypertextLink = 
          new TXTextControl.HypertextLink(hyperlink.Text, hyperlink.Target);
        tx.HypertextLinks.Add(hypertextLink);

        // save the content and return to the event arguments
        byte[] data;
        tx.Save(out data, TXTextControl.BinaryStreamType.InternalUnicodeFormat);
        e.MergedField = data;
      }
    }
  }

}