<tab id="testtab">
            <label>Test settings</label>
        </tab>
        <section id="testsection" type="text" showInDefault="1">
            <label>Test Configuration</label>
            <tab>testtab</tab>
            <resource>Namespace_Modulename::system_config</resource>
            <group id="settings" type="text"  sortOrder="10" showInDefault="1">
                <label>Credentials</label>
                <field id="key" translate="label" type="text" sortOrder="30" showInDefault="1">
                    <label>Key</label>
                </field>
            </group>
        </section>

public function configDirective($construction)
{
    $configValue = '';
    $params = $this->getParameters($construction[2]);
    $storeId = $this->getStoreId();
    if (isset($params['path']) && $this->isAvailableConfigVariable($params['path'])) {
        $configValue = $this->_scopeConfig->getValue(
            $params['path'],
            MagentoStoreModelScopeInterface::SCOPE_STORE,
            $storeId
        );
    }
    return $configValue;
}

private function isAvailableConfigVariable($variable)
{
    return in_array(
        $variable,
        array_column($this->configVariables->getData(), 'value')
    );
}

<type name="MagentoEmailModelSourceVariables">
    <plugin name="add-test-credentials" type="[Vendor][Module]PluginModelSourceVariablePlugin" />
</type>

<?php
namespace [Vendor][Module]PluginModelSource;

use MagentoEmailModelSourceVariables;

class VariablePlugin
{
    /**
     * @param Variables $subject
     * @param $data
     * @return array
     */
    public function afterGetData(Variables $subject, $data)
    {
        $data[] = [
            'value' => 'testsection/settings/key',
            'label' => __('Credentials for test settings') //give it a label that suits you best. This will be listed in the insert custom var popup in the cms pages and blocks (see image below)
        ];
        return $data;
    }
}