Hello Ashish,
You can use below code to solve your issue. Please reward if it helps.
***************************************************************************************
REPORT ZDEMO_HELP.
TABLES: T005, KNA1.
* TYPE DECLARATION
TYPES: BEGIN OF TY_KNA1, " INCLUDE FIELDS WHAT YOU WANT IN HELP LIST
KUNNR TYPE KNA1-KUNNR,
LAND1 TYPE KNA1-LAND1,
NAME1 TYPE KNA1-NAME1,
NAME2 TYPE KNA1-NAME2,
ORT01 TYPE KNA1-ORT01,
END OF TY_KNA1.
* INTERNAL TABLE AND WORK AREA DECLARATION
DATA: IT_KNA1 TYPE TABLE OF TY_KNA1,
WA_KNA1 TYPE TY_KNA1.
* DECLARATION
PARAMETERS: COUNTRY LIKE T001-LAND1, " TO SELECT COUNTRY
CUSTOMER LIKE KNA1-KUNNR. " TO SELECT CUSTOMER BASED COUNTRY
*AT SELECTION-SCREEN VALUE REQUEST FOR CUSTOMER .
AT SELECTION-SCREEN ON VALUE-REQUEST FOR CUSTOMER.
DATA: IT_DYNPFIELDS TYPE STANDARD TABLE OF DYNPREAD,
WA_DYNPFIELDS TYPE DYNPREAD.
WA_DYNPFIELDS-FIELDNAME = 'COUNTRY'.
APPEND WA_DYNPFIELDS TO IT_DYNPFIELDS.
CALL FUNCTION 'DYNP_VALUES_READ'
EXPORTING
DYNAME = SY-REPID
DYNUMB = SY-DYNNR
TABLES
DYNPFIELDS = IT_DYNPFIELDS.
READ TABLE IT_DYNPFIELDS INTO WA_DYNPFIELDS WITH KEY FIELDNAME = 'COUNTRY'.
IF SY-SUBRC = 0.
COUNTRY = WA_DYNPFIELDS-FIELDVALUE.
ENDIF.
* SELECT QUIRES FOR CUSTOM FIELDS
SELECT KUNNR LAND1 NAME1 NAME2 ORT01
INTO CORRESPONDING FIELDS OF TABLE IT_KNA1 FROM KNA1
WHERE LAND1 EQ COUNTRY.
* FUNCTION MODULE TO POPULATE F4 HELP
CALL FUNCTION 'F4IF_INT_TABLE_VALUE_REQUEST'
EXPORTING
RETFIELD = 'KUNNR'
DYNPPROG = SY-REPID
DYNPNR = SY-DYNNR
DYNPROFIELD = 'CUSTOMER '
* WINDOW_TITLE = 'Select Customer'
VALUE_ORG = 'S'
* MULTIPLE_CHOICE = ' '
* IMPORTING
* USER_RESET =
TABLES
VALUE_TAB = IT_KNA1.
Thanks
Piyush Gupta