extra check on matnr in order-items VA01, VA02

Hello dear friends

In the order-items of VA01 and VA02 i have to build an extra check on the MATNR to accept it (or not accpet it with a message).

I have already build the function to check.

Can someone tell where i can build-in this function This function only needs customer number, sales organisation and matnr. and must be activated directly after entering an new itemnr in an itemline.

Hope someone can help me.

Regards, Frank Bernsen