Thank you for your answer but i've already check these methods but i can't find where to add my new field. it will be really helpful if you give me examples.

Thank you in advance !

Hi,

From the _run_buy method, another method _prepare_purchase_order_line_from_procurement is called, from there you can see the function _prepare_purchase_order_line to update from there a function called values into purchase. Do check the method, it may help you.

Hope it helps