i didnt looked for bapi.
just implicit enhancement solved the purpose.
check this:
Main Program SAPLSZA1
Include LSZA1F12
There is a subroutine
FORM set_dialog_flag_regional_data.
In the End of this form..i created a implicit enhancement.
ENHANCEMENT 2 ZEI_XD01_LSZA1F12. "active version
BREAK ppuser.
* u can place more tabs in SY-UCOMM, TAB03, TAB04-TAB07.
if addr1_data-REGION is INITIAL and sy-ucomm eq 'TAB02'.
message 'Region Field Can Not Be Empty' type 'E'.
ENDIF.
ENDENHANCEMENT.