I am creating a Purchase Order form, in which I have an Auto Number field, for the number of each order. There is also a Lookup field with the companies we take orders from. Each company has a different count for the purchase orders. I would like the auto number field to relate the order number for each company.

Example:

Company A = current order 1

company B = current order 234

Company C = current oder 123

So, when the user selects in the lookup fiel company A I wold like the Auto Number field to be 2. When the user selects company B I would like the auto number to be 233 and so on.

How can I do this?