How to give a CSS class via XML?

There is a link in header portion called quick order, I need to add css class for that link but there is no phtml file for that link so I need to add it via xml. Below is the xml file from vendor.

<referenceContainer name="header.panel">
            <block class="MagentoFrameworkViewElementHtmlLinks" name="b2blinks" before="top.links">
                <arguments>
                    <argument name="css_class" xsi:type="string">header links</argument>
                </arguments>
                <block class="MagentoQuickOrderBlockLink" name="quick_order_link" ifconfig="btob/website_configuration/quickorder_active" after="-"/>
            </block>
        </referenceContainer>

How can give css class for quick order link?