manufacture/mrp_subcontracting_purchase/views/purchase_order_views.xml

<?xml version="1.0" encoding="utf-8" ?>
<odoo>
    <record id="po_form_mrp_subcontracting_purchase" model="ir.ui.view">
        <field name="name">
            purchase.order.inherited.form.mrp.subcontracting.purchase
        </field>
        <field name="model">purchase.order</field>
        <field name="inherit_id" ref="purchase.purchase_order_form" />
        <field name="arch" type="xml">
            <button name="action_view_picking" position='before'>
                <button
                    class="oe_stat_button"
                    name="action_view_subcontracting_resupply"
                    type="object"
                    icon="fa-truck"
                    attrs="{'invisible': [('subcontracting_resupply_picking_count', '=', 0)]}"
                    groups="stock.group_stock_user"
                >
                    <div class="o_field_widget o_stat_info">
                        <span class="o_stat_value">
                            <field name="subcontracting_resupply_picking_count" />
                        </span>
                        <span class="o_stat_text">Resupply</span>
                    </div>
                </button>
            </button>
        </field>
    </record>
</odoo>