user exit for VA01.

My scenaio is like ths :

If The company has 4 brands of product and according to their sales policy customer must order all 4 brands then only they accept the sales order.

In VA01 when we create sales order, when we enter material in 2nd creen, If all 4 brand is entered then only we can proceed for next step , if one of the brand is missing it will give error mease as ' all brand is not orderd'.

Also exit name is MV45AFZZ - USEREXIT_MOVE_FIELD_TO_VBAP.

Can anybody explain me how to do with coding as material will be entered in table control in VA01.

thanks

naresh