odoo16/addons/mrp_subcontracting_purchase/views/purchase_order_views.xml

<?xml version="1.0" encoding="utf-8"?>
<odoo>
    <record id="purchase_order_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">
            <xpath expr="//div[hasclass('oe_button_box')]/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>
            </xpath>
        </field>
    </record>
</odoo>