pos/pos_require_product_quantity/static/src/xml/templates.xml

<?xml version="1.0" encoding="utf-8"?>
<odoo>
    <data>
        <template id="assets_backend" name="pos_require_product_quantity_extension_assets" inherit_id="web.assets_backend">
          <xpath expr="." position="inside">
              <script type="text/javascript"
                      src="/pos_require_product_quantity/static/src/js/pos.js">
              </script>
          </xpath>
        </template>
    </data>
</odoo>