CJ20N transaction - sap sceen field validation version ECC 6.0

Hello Friends,

For screen 2701 in SAP version ECC 6.0, how do I make field 'Recipient' a required field without doing a core mod? There is a user exit ZXCN2U06, which I added codes to validate this field. However, the warning pops up but the screen to enter the data doesn't appear. We applied this to 4.6 by core mod but don't want to do core mod on 6.0.

Thanks