Here is a short example of using list boxes on selection screens:

 PROGRAM zcmtest01. 
 TYPE-POOLS: vrm. 
 DATA: name TYPE vrm_id, 
       list TYPE vrm_values, 
       value LIKE LINE OF list. 

PARAMETERS: ps_parm(10) AS LISTBOX VISIBLE LENGTH 10.
AT SELECTION-SCREEN OUTPUT. 
  name = 'PS_PARM'. 
  value-key = '1'.
  value-text = 'Line 1'.
  APPEND value TO list.
  value-key = '2'. 
  value-text = 'Line 2'.
  APPEND value TO list. 
  CALL FUNCTION 'VRM_SET_VALUES' 
    EXPORTING id     = name 
              values = list. 
              
START-OF-SELECTION. 
  WRITE: / 'Parameter:', ps_parm.