How do you add a tax rate area to a specific item?

I am trying to add a specific tax rate area to an item and then get it to pull through onto a sales order, overwriting the tax rate area defaulting from the customer master.

I can see that in the tax rate area set up, there is a field for an item number and I have added the number here but it does not pull though onto the sales order line.

Does anyone know how to do this?

Thanks in advance!