contract/agreement_legal_sale/views/sale_order.xml

<?xml version="1.0" encoding="utf-8"?>
<odoo>
    <!-- Copyright 2019 Open Source Integrators
         License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl). -->

    <!-- Agreement Form View -->
    <record id="sale_order_agreement_form_view" model="ir.ui.view">
        <field name="name">sale.order.agreement.form.view</field>
        <field name="model">sale.order</field>
        <field name="inherit_id" ref="sale.view_order_form"/>
        <field name="arch" type="xml">
            <field name="client_order_ref" position="after">
                <field name="agreement_id"
                       readonly="1"
                       attrs="{'invisible': [('agreement_id', '=', False)]}"/>
            </field>
            <field name="partner_id" position="after">
                <field name="agreement_template_id"/>
            </field>
        </field>
    </record>

</odoo>