SSRS Multi value parameter on Enterprise Portal

Wondering how you pass a multi value parameter to a SSRS report in the AXReportViewer control?  I have tried passing it as a comma delimited string, but that doesn't seem to work.  Below is the code I am using to pass this information.  It is using the standard AddParameters method on the control.  It seems to work if i only pass a single value but passing a comma delimited list doesn't work.  Wondering if anyone has done this before or not. 

       Dictionary<string, object> parms = new Dictionary<string, object>();

       IAxContext axContext = AxContextHelper.FindIAxContext(this); ;

       parms.Add(BookedDateStartParamName, DateTime.Parse(this.TextboxFromDate.Text, axContext.CultureInfo).ToString("G"));

       parms.Add(BookedDateEndParamName, DateTime.Parse(this.TextboxToDate.Text, axContext.CultureInfo).ToString("G"));

       parms.Add(RegionParamName, RegionTextBox.Text);

       parms.Add(BranchParamName, BranchTextBox.Text);

       this.SalesBookingsReport.AddParameters(parms);