I eventually discovered how to do it.
In the above example I just remove
msg.ROSpec.ROReportSpec.TagReportContentSelector = new PARAM_TagReportContentSelector();
and replace it with
PARAM_TagReportContentSelector reportContent = new PARAM_TagReportContentSelector();
reportContent.EnableAntennaID = true;
msg.ROSpec.ROReportSpec.TagReportContentSelector = reportContent;