STEP 1 : FIELD CREATION :
CREATE A FIELD IN VBAK ( LETS SAY ZZFIELD1) - INCLUDE A STRUCTURE AND ADD A NEW FIELD , ACTIVATE BOTH YOUR STRUCTURE AND VBAK.

STEP 2 .GO TO THE SCREEN: (SE51)
PROGRAM NAME: SAPMV45A
SCREEN NUMBER : 8309

GO TO SCREEN LAYOUT AND CREATE A FIELD FOR YOU AND ASSIGN THE FIELD TO ZZFIELD1.
IN FLOW LOGIC:
PBO – CREATE A MODULE LETS SAY ; MODULE MODULE ZZSETSCREEN.
DOUBLE CLICK THE MODULE , U WILL LAND AT PROGRAM MV45AOZZ AND WRITE THE BELOW CODE INIT

STEP 3 : WRITE CODE IN MV45AOZZ
MODULE ZZSETSCREEN OUTPUT.
* TO MAKE THE FIELD NON-EDITABLE ONLY FOR DISPALY MODE
LOOP AT SCREEN.
IF SY-TCODE = 'VA03'.
SCREEN-INPUT = 0.
SCREEN-OUTPUT = 1.
MODIFY SCREEN.
ELSE.
IF VBAK-ZZFIELD1 IS INITIAL.
SCREEN-REQUEST = 1.
MODIFY SCREEN.
ENDIF.
ENDIF.
ENDLOOP.

ENDMODULE.

STEP 4 : WRITE CODE IN MV45AIZZ
PAI: FIELD VBAK-ZZFIELD1
CREATE A MODULE IN PAI , LETS SAY –
FIELD VBAK-ZZFIELD1 MODULE ZZCHECKFIELD1 ON INPUT , AGAIN DOUBLE CLICK THE MODULE , U WILL REACH MV45AIZZ
WRITE A CODE LIKE THIS ( INCLUDE YOUR LOGIC )
MODULE ZZCHECKFIELD1 INPUT.
IF NOT VBAK-ZZTEST1 IS INITIAL.
DO THE CHECK WHATEVER U WANT, RAISE ERROR MESSAGE IF YOU NEED ETC ETC
ENDIF.

ENDMODULE