You need to pass an additional table parameter field_tab. Please refer below.
PARAMETERS: p_sales TYPE vbak-vbeln.
TYPES: BEGIN OF st_vbak,
vbeln TYPE vbak-vbeln,
name1 TYPE kna1-name1,
END OF st_vbak.
DATA: it_tvbak TYPE TABLE OF st_vbak,
wa_tvbak TYPE st_vbak,
it_field_tab TYPE TABLE OF dfies,
wa_field_tab TYPE dfies.
AT SELECTION-SCREEN ON VALUE-REQUEST FOR p_sales.
SELECT vbak~vbeln kna1~name1 AS customername
FROM vbak INNER JOIN kna1 ON vbak~kunnr = kna1~kunnr
INTO TABLE it_tvbak UP TO 10 ROWS.
*** Sales Doc
wa_field_tab-tabname = 'VBAK'.
wa_field_tab-fieldname = 'VBELN'.
APPEND wa_field_tab TO it_field_tab.
*** Customer Name
CALL FUNCTION 'DDIF_FIELDINFO_GET'
EXPORTING
tabname = 'KNA1'
lfieldname = 'NAME1'
IMPORTING
dfies_wa = wa_field_tab
EXCEPTIONS
OTHERS = 3.
wa_field_tab-fieldtext = 'Customer Name'.
wa_field_tab-reptext = 'Customer Name'.
APPEND wa_field_tab TO it_field_tab.
CALL FUNCTION 'F4IF_INT_TABLE_VALUE_REQUEST'
EXPORTING
retfield = 'VBELN'
dynpprog = sy-repid
dynpnr = sy-dynnr
dynprofield = 'P_SALES'
value_org = 'S'
TABLES
value_tab = it_tvbak
field_tab = it_field_tab.