Need User-Exit / Badi For CO11N selection screen

Hello Gurus,

I need to add an extra functionality in the selection screen of the Tcode CO11N .Can you suggest some user exit or badi for the same .

The Functionality is : We need to make the field 'Reason' as mandatory whenever there is an entry in the field 'Scarp' .

Waiting for your valuable suggestion on this .

Regards,

Abhijit Mandal