Validation in T/C ME23N

hi

I have a requirement where I have to validate two fields, one of them is on Item overview, column "Requisitioner" field AFNAM and the other one is on Item "Account assignment" Cost Center field KOSTL. Which BADI should I use to make this validation of the list below, I will appreciate any help.

ME_PROCESS_PO
ME_PROCESS_COMP
ME_PO_SC_SRV
ME_MMPUR_EINM_CUST
ME_MMPUR_EINM
ME_HOLD_PO
ME_GUI_PO_CUST
ME_FIELDSTATUS_STOCK
ME_DEFINE_CALCTYPE
ME_COMMTMNT_REQ_RE_C
ME_COMMTMNT_REQ_RELE
ME_COMMTMNT_PO_REL_C
ME_PROCESS_PO_CUST
SMOD_MRFLB001
MM_EDI_ORDERS_OUT
MM_EDI_DESADV_IN
MM_EDI_DELFOR_OUT
ME_TRIGGER_ATP
ME_TAX_FROM_ADDRESS
ME_REQ_POSTED
ME_REQ_OI_EXT
ME_RELEASE_CREATE
ME_PURCHDOC_POSTED
ME_PROCESS_REQ_CUST
ME_PROCESS_REQ
ME_COMMTMNT_PO_RELEV
ME_BSART_DET
ME_BAPI_PR_IBS
ME_BAPI_PR_CUST
ME_BAPI_PR_CREATE_02
ME_BAPI_PR_CREATE_01
ME_BAPI_PO_IBS
ME_BAPI_PO_CUST
ME_BAPI_PO_CREATE_02
ME_BAPI_PO_CREATE_01
ME_ACTV_CANCEL_PO
MEGUI_LAYOUT
EXTENSION_US_TAXES
ME_CCP_ACTIVE_CHECK
ME_COMMITMENT_STO_CH
ME_CIP_REF_CHAR
ME_CIP_ALLOW_CHANGE
ME_CIN_MM06EFKO
ME_CIN_LEINRF2V
ME_CIN_LEINRF2R
ME_CHECK_SOURCES
ME_CHECK_ALL_ITEMS
ME_CHANGE_OUTTAB
ME_CHANGE_CHARACTER
ME_CCP_DEL_DURATION
ME_CCP_BESWK_AUTH_CH

Thanks in advance.