How to read Report selection screen field values before PAI field transport

Function: DYNP_VALUES_READ

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How to got the field value when you still in selection screen manipulating, example code as below:

AT SELECTION-SCREEN ON VALUE-REQUEST FOR p_vbeln.
*--- Update P_VSTEL
  dyfields-fieldname = 'P_VSTEL'.
  APPEND dyfields.
  CALL FUNCTION 'DYNP_VALUES_READ'
    EXPORTING
      dyname     = sy-cprog
      dynumb     = sy-dynnr
    TABLES
      dynpfields = dyfields.
  READ TABLE dyfields INDEX 1.

  IF dyfields-fieldvalue IS INITIAL.
    MESSAGE i899 WITH 'Please enter Shipping Point first!'.
    EXIT.
  ELSE.
    CLEAR: gr_vstel,  gr_vstel[],
           gt_lipov,  gt_lipov[],
           gt_dn,     gt_dn[].
    gr_vstel-sign   = 'I'.
    gr_vstel-option = 'EQ'.
    gr_vstel-low    = dyfields-fieldvalue.
    APPEND gr_vstel.
  ENDIF.

  CALL FUNCTION 'WS_LM_DATA_SELECTION'
    EXPORTING
      if_proctype     = 'W'
    TABLES
      it_vstel        = gr_vstel
      ct_postab       = gt_lipov
    EXCEPTIONS
      no_data_found   = 1
      wrong_proc_type = 2
      OTHERS          = 3.
  IF sy-subrc <> 0.
* MESSAGE ID SY-MSGID TYPE SY-MSGTY NUMBER SY-MSGNO
*         WITH SY-MSGV1 SY-MSGV2 SY-MSGV3 SY-MSGV4.
  ENDIF.

  LOOP AT gt_lipov.
    CLEAR gt_dn.
    MOVE-CORRESPONDING gt_lipov TO gt_dn.
    SELECT SINGLE name1 INTO gt_dn-name1
      FROM kna1
      WHERE kunnr = gt_dn-kunnr.
    APPEND gt_dn.
  ENDLOOP.

  CALL FUNCTION 'F4IF_INT_TABLE_VALUE_REQUEST'
    EXPORTING
      retfield    = 'VBELN'
      dynpprog    = sy-repid
      dynpnr      = sy-dynnr
      dynprofield = 'P_VBELN'
      value_org   = 'P'
    TABLES
      value_tab   = gt_dn
      field_tab   = gt_dfies2.