system.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Config:etc/system_file.xsd">
    <system>
        <section id="sales">
            <group id="ordercomments" translate="label" type="text" sortOrder="110" showInDefault="1" showInWebsite="1" showInStore="0">
                <label>Order Comment</label>
                <field id="show_in_account" translate="label" type="select" sortOrder="20" showInDefault="1" showInWebsite="1" showInStore="0" canRestore="1">
                    <label>Show comments in customer account</label>
                    <source_model>Magento\Config\Model\Config\Source\Yesno</source_model>
                </field>
            </group>
        </section>
    </system>
</config>