I think I did... following the example given on the API doc page, the request is set up like so:
MailboxDumpRequest mailboxDumpRequest = new MailboxDumpRequest();
mailboxDumpRequest.BeginDate = DateTime.Now.Subtract(new TimeSpan(60, 0, 0, 0));
mailboxDumpRequest.EndDate = DateTime.Now;
mailboxDumpRequest.IncludeDeleted = false;
mailboxDumpRequest.SearchQuery = "in:chat";
mailboxDumpRequest.PackageContent = MonitorLevel.FULL_MESSAGE;