Rendere una dynpro sensibile al click su un checkbox

CONSTANTS: gc_blank_time TYPE char6 VALUE '      '.

PARAMETERS: p_date_l TYPE vekp-erdat.
PARAMETERS: p_date_h TYPE vekp-erdat.
PARAMETERS: p_time_l TYPE vekp-eruhr.
PARAMETERS: p_time_h TYPE vekp-eruhr.
PARAMETERS: p_timest AS CHECKBOX USER-COMMAND com001.

INITIALIZATION.
  p_time_l = gc_blank_time.
  p_time_h = gc_blank_time.

AT SELECTION-SCREEN OUTPUT.
  CHECK p_timest IS INITIAL.
  LOOP AT SCREEN.
    IF p_date_l IS INITIAL AND screen-name = 'P_DATE_L'.
      screen-required = 2.
    ENDIF.
    IF p_date_h IS INITIAL AND screen-name = 'P_DATE_H'.
      screen-required = 2.
    ENDIF.
    IF p_time_l = gc_blank_time AND screen-name = 'P_TIME_L'.
      screen-required = 2.
    ENDIF.
    IF p_time_h = gc_blank_time AND screen-name = 'P_TIME_H'.
      screen-required = 2.
    ENDIF.
    MODIFY SCREEN.
  ENDLOOP.