Ms Access Purchase order with Sub form and auto line item number

[EDR]

Hi,

With have created a purcahse oder form with auto number "POYYMM####"
(PO06040010).
Each month / year the number reset at number 1.
Part of this table is all the purchase order header details. (purcase
order table)
We also have a subform for the purchase order item details. and a table
linked to the (purcase order table) vias the purchase order number. the
subform table is "the purchase order item details" table.
We are trying to implement an auto line item number for each line item
of the subform.
we try many ways....but can you help thank.

Olivier