Focus on product code field in shopping cart add option

[Guest dook]

Some of my clients enter their orders by product code but find it inefficient beacuse they have to use the mouse to click on the field every time before they start keying in the product code.

Can anyone tell me how to place the focus on the field, I have tried the following javascipt but not sure of location or document name etc.

<script>

setTimeout(’document.cart.productCode.focus()’,500);

</script>

here's hoping