purchase_order_view.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- Copyright 2016 Pedro M. Baeza <pedro.baeza@tecnativa.com>
     License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl). -->
<odoo>

    <record id="purchase_order_form" model="ir.ui.view">
        <field name="model">purchase.order</field>
        <field name="inherit_id" ref="purchase.purchase_order_form"/>
        <field name="arch" type="xml">
            <xpath expr="//field[@name='order_line']" position="before">
                <div class="oe_button_box" name="button_box">
                    <button name="%(action_purchase_manage_variant)d"
                            type="action"
                            string="Add or Modify Variants"
                            class="oe_edit_only"
                            states="draft,sent"
                    />
                </div>
            </xpath>
            <xpath expr="//field[@name='order_line']//tree" position="inside">
                <field name="product_tmpl_id_purchase_order_variant_mgmt" invisible="1"/>
                <field name="state_purchase_order_variant_mgmt" invisible="1"/>
                <field name="product_attribute_value_ids" invisible="1"/>
                <!-- Not working until https://github.com/odoo/odoo/pull/13558 -->
                <!-- Also https://github.com/odoo/odoo/pull/13635 is needed for correct template selection -->
                <button name="%(action_purchase_manage_variant)d"
                        type="action"
                        string="Modify Variants"
                        icon="fa-th"
                        class="oe_edit_only"
                        context="{'default_product_tmpl_id': product_tmpl_id_purchase_order_variant_mgmt}"
                        attrs="{'invisible': ['|', ('state_purchase_order_variant_mgmt', 'not in', ('draft', 'sent')), ('product_attribute_value_ids', '=', [])]}"
                />
            </xpath>
            <xpath expr="//field[@name='order_line']" position="attributes">
                <attribute name="options">{'reload_on_button': true}</attribute>
            </xpath>
        </field>
    </record>

</odoo>