Form+Table, 2nd Autonumber Help

I have a table that stores orders in. There is a form to input data into this table, that I choose the products from a products table (using a subform) for the order. Once I submit the form, it submits several rows according to how many products I chose (a row for each product and the order) to the orders table.

The orders table has a autonumber primary key ID, so each of these rows has a different ID number.

I also want a column to store an order number (field: OrderNo) that is the same for each row (each product) for this order in question and not be different, allowing me to do a query for that order number and see all the products under it.

So I need the form to lookup the last highest order number from the OrderNo field, add +1 then when I add a product, it gives the SAME number to each one.

I hope this makes sense and I hope you can explain to how I do this? :)

Thanks,
Nick.