Add parameters to the folder is simple, you just need to right click and then click on add, I think your question is more focused on how to send the parameters to the report. To do this you first need to instantiate an array of the ReportParameter class as I show you below:
ReportParameter[] reportParameters = new ReportParameter[1];
reportParameters[0] = new ReportParameter("NombreParametro", "VALOR DE TU PARAMETRO", false);
Then if you have your report in a ReportViewer you must access the LocalReport property and then add the parameters:
reportViewer1.LocalReport.SetParameters(reportParameters);
reportViewer1.RefreshReport();
In case you do not have a ReportViewer, access the property of your report directly:
TuReporte..SetParameters(reportParameters);
I hope it will be useful for you.