how to config or enhancements a popup in VL02N ?

Dear All:

I have a program for VL02N to Enhancements:

When user input the DN number in VL02N screen and press the enter key,user want to popup a window to show the DN item material current stock.

I find a user-exit :u2019 MV50AFZu2019 and form u2018USEREXIT_READ_DOCUMENTu2019 can realize it.But when i used WINDOW STARTING AT ,this code canu2019t work .

And when I used message code,the same alse canu2019t to popup a window.

How can I do,Please help me !

Thanks

Sun