Hi,

1. Create Xml file (purchase_order_views.xml)

<?xml version="1.0" encoding="utf-8"?>
<odoo>
<!-- Inherit Form View to Modify it -->
<record id="view_inherit_purchase_order_form" model="ir.ui.view">
<field name="name">purchase.order.form</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')]" position="inside">
<button name="action_preview_purchase_order"
                    type="object"
                    class="oe_stat_button"
                    icon="fa-globe icon">
                <div class="o_field_widget o_stat_info">
<span class="o_stat_text">Vendor</span>
<span class="o_stat_text">Preview</span>
                </div>
            </button>
        </xpath>
    </field>
</record>
</odoo>

2. Create a Python file(purchase_order)
class PurchaseOrder(models.Model):
    _inherit = 'purchase.order'

    def action_preview_purchase_order(self):
        self.ensure_one()
        return {
            'type': 'ir.actions.act_url',
            'target': 'self',
            'url': self.get_portal_url(),
        }

Hope it helps