AT SELECTION-SCREEN ON VALUE-REQUEST FOR p_werks , P_werks is parameter variable.

REPORT  YKC_SEARCH_HELP.
*internal table made to populate the value of werks when pressing f4
DATA: BEGIN OF IT_FINAL OCCURS 0,
      WERKS TYPE MARC-WERKS,
      END OF IT_FINAL.
data: IT_RETURN LIKE DDSHRETVAL OCCURS 0 WITH header line.
parameters: p_werks(10) type c.
AT SELECTION-SCREEN ON VALUE-REQUEST FOR p_werks.
  select  werks from marc
  into table IT_FINAL.
  CALL FUNCTION 'F4IF_INT_TABLE_VALUE_REQUEST'
    EXPORTING
*   DDIC_STRUCTURE         = ' '
      RETFIELD               = 'WERKS'   "field of internal table
     VALUE_ORG              = 'S'
    TABLES
      VALUE_TAB              = IT_FINAL
*   FIELD_TAB              =
     RETURN_TAB             = IT_R

ETURN

            .

 WRITE IT_RETURN-FIELDVAL TO P_WERKS.

  REFRESH IT_FINAL.