Identifying changes in the custom screen added to Delivery

Hello,

I have added a custom screen in delivery transaction(VL02n) at header level. I used BADI LE_SHP_TAB_CUST_HEAD to do that. Now if user comes and enters some data on the custom screen, then goes to menu option Subsequent processing-->Create Transfer Order, user is not prompted with a pop up screen to save changes.

It happens with SAP Standard screens for any data changes. How do we achieve the same functionality to prompt a popup message for any changes in the custom screen and save the data?

Thanks

Anil