contract/agreement_sale/views/sale_order.xml

<?xml version="1.0" encoding="utf-8"?>
<odoo>
    <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"
                    domain="[('partner_id', 'child_of', commercial_partner_id), ('domain', '=', 'sale')]"
                context="{'default_partner_id': commercial_partner_id, 'default_domain': 'sale'}"/>
            </field>
            <field name="partner_id" position="after">
                <field name="agreement_type_id"
                       groups="agreement.group_use_agreement_type"
                       />
            </field>
        </field>
    </record>

    <record id="view_sales_order_filter" model="ir.ui.view">
        <field name="name">agreement.sale.order.search</field>
        <field name="model">sale.order</field>
        <field name="inherit_id" ref="sale.view_sales_order_filter"/>
        <field name="arch" type="xml">
            <field name="order_line" position="after">
                <field name="agreement_id" />
            </field>
            <group expand="0" position="inside">
                <filter string="Agreement" name="agreement_groupby" context="{'group_by': 'agreement_id'}"/>
            </group>
        </field>
    </record>

</odoo>