AddCustomConfigVariable

config.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Store:etc/config.xsd">
    <default>
        <general>
            <store_information>
                <support_email_address>support@magevision.com</support_email_address>
            </store_information>
        </general>
    </default>
</config>

di.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
        <type name="Magento\Variable\Model\Config\Structure\AvailableVariables">
            <arguments>
                <argument name="configPaths" xsi:type="array">
                    <item name="general/store_information" xsi:type="array">
                        <item name="general/store_information/support_email_address" xsi:type="string">1</item>
                    </item>
                </argument>
            </arguments>
        </type>
</config>

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="general">
            <group id="store_information">
                <field id="support_email_address" translate="label" sortOrder="65" showInDefault="1" showInWebsite="1">
                    <label>Support Email Address</label>
                </field>
            </group>
        </section>
    </system>
</config>