Making Internal Order Mandatory in FB75 for Particular User

Hi,

In FB75 Item Details, I have to make 'Order Number' Number as Mandatory for Particular User ID. I am  trying to Validate in OB28. But it not working.

BKPF-BUKRS = 'NPIN' AND SYST-TCODE = 'FB75' AND

BKPF-SNAME = 'xxx

BSEG-AUFNR <> '''

Thanks

Muthu