Hi,

In the “purchase.purchase_order_form” view, there are two buttons named "button_confirm."  You have to give the confirm attribute to both buttons to achieve your requirement; see below.

<button name="button_confirm" type="object" invisible="state != 'sent'" string="Confirm Order" confirm="Are you sure you want to confirm the purchase?" context="{'validate_analytic': True}" class="oe_highlight" id="bid_confirm" data-hotkey="q"/>

<button name="button_confirm" type="object" invisible="state != 'draft'" confirm="Are you sure you want to confirm the purchase?" context="{'validate_analytic': True}" string="Confirm Order" id="draft_confirm" data-hotkey="q"/>

Hope it helps