web/web_m2x_options/views/view.xml

<?xml version="1.0" encoding="utf-8"?>
<odoo>

    <template id="assets_backend" name="m2x options assets"
              inherit_id="web.assets_backend">
        <xpath expr="." position="inside">
            <script type="text/javascript"
                    src="/web_m2x_options/static/src/js/form.js"></script>
        </xpath>
    </template>

</odoo>