To get selection parameter

use FM: RS_REFRESH_FROM_SELECTOPTIONS

ex:
 CALL FUNCTION 'RS_REFRESH_FROM_SELECTOPTIONS'
   EXPORTING
     curr_report     = 'ZTEST_CS15'
   TABLES
     selection_table = it_seltab
   EXCEPTIONS
     not_found       = 1
     no_report       = 2
     OTHERS          = 3.