contract/contract_fieldservice/views/sale_view.xml

<odoo>

    <record id="view_order_form_inherit_contract_fieldservice" model="ir.ui.view">
        <field name="name">sale.order.form</field>
        <field name="model">sale.order</field>
        <field name="inherit_id" ref="sale.view_order_form"/>
        <field name="arch" type="xml">
            <field name="fsm_location_id" position="attributes">
                <attribute name="domain">[('contract_ids', 'in', parent.contract_id)]</attribute>
            </field>
        </field>
    </record>

</odoo>