Very urgent regd Validation

Hi all,

I have a requirement where, while posting a document using F-02, it should display a message if MATNR is blank,

I have set MATNR as optional in SPRO,

it should post all documents with/without MATNR expect for G/L account '0000597656'.

so if G/L Account is '0000597656'.

and if Matnr = ' ' .

then it should display a error msg and stop posting,

I am trying the same using Tcode OB28,

i gave prerequistie : BSEG-HKONT = '0000597656'.

and in Check : BSEG-MATNR = ' '.

Message : Enter matnr,

please let me know if this process is correct, or what should I enter in Prerequistie and check coloumns,

its very urgent.

Many thanks,