Auto fill the goods' storage location in sales order item

Hi,

I have a issue,When I use VA01 to create a sales order,I want to let the system auto fill the material storage location when I enter the material no. for the sales ITEM.

How to do?I want use the userexit to do,the program 'MV45ATZZ' but I couldn't find where I can add my code,Whether have other better way to do?

Thanks