To append a default value for selection-screen.
2 ways.
To code it after in the event

INITIALIZATION.
RSK_RANGE-SIGN = 'I'.
RSK_RANGE-OPTION = 'BT'.
RSK_RANGE-LOW = '00'.
RSK_RANGE-HIGH = '99'.
APPEND RSK_RANGE.
or

SELECT-OPTIONS: S_DATE  FOR SY-DATUM OBLIGATORY  NO-EXTENSION
                DEFAULT '20121101'
                     TO '20121231'
                 OPTION  BT
                   SIGN  I .