Select option value only from F4 help and custom search help

Hi,

  I have created a custom search help for a field like VBELN, the custom search help is made from a database view, I have assign that search help to selection screen field, Now i want that user should enter the value in the selection parameter only from F4 help, I have check some threads but in that at selection-screen on value request concept is use, then first the values are selected in the internal table and return with F4IF*  function modulw, my code is as below.

   PARAMETERS : VBELN TYPE VBELN MATCHCODE OBJECT ZMC_SD_SDOC MODIF ID SON.


AT SELECTION-SCREEN OUTPUT.

  LOOP AT SCREEN.
    IF SCREEN-GROUP1 = 'SON'.
       SCREEN-INPUT = '0'.
       MODIFY SCREEN.
    ENDIF.
  ENDLOOP.


START-OF-SELECTION.

but now when i clcik on search help on selection screen, i am getting the search help window but value are not getting return to VBELN.

REGARDS,

zafar