i've solve this myself;
string vardisid = frmdis.txt_DiscardID.Text;
string selectFormula = "({ItemDiscard.DiscardID}= '" + vardisid + "')";
crystalReportViewer1.SelectionFormula = selectFormula;
REPORTS.Rpt_Discard gdrreport = new Rpt_Discard();
crystalReportViewer1.ReportSource = gdrreport;
i introduce temporary object first to pass, other form textbox value:
string vardisid = frmdis.txt_DiscardID.Text;
then this "vardisid" use for report selection formula text: bingo, it work like charm.
string selectFormula = "({ItemDiscard.DiscardID}= '" + vardisid + "')";
thanks!