Field Exit and User Exit

MM42
Key in article and select Logistics, execute then hot enter.
Select Accounting 

Data Element: BKLAS

Program: SAPLMGD2

Screen: 2802

Data Element: VERPR

Run program: RSMODPRF
Execute
FIELD_EXIT_BKLAS
Create Field Exit
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User exit

Program: SAPLMGMW
Screen:  3000

Next to find the package
SE38 pass the program SAPLMGMW then select attribute
The package: MGW

Now find the User exit
SMOD then go to menu utilities/find

fill in MGW then execute

MGW0001
MGW0002