system.xml

<?xml version="1.0"?>
<!--
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
-->
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Config:etc/system_file.xsd">
    <system>
        <section id="catalog">
            <group id="search">
                <!-- Elasticsearch 6.0+ -->
                <field id="elasticsearch6_server_hostname" translate="label" type="text" sortOrder="71"
                       showInDefault="1" showInWebsite="0" showInStore="0">
                    <label>Elasticsearch Server Hostname</label>
                    <depends>
                        <field id="engine">elasticsearch6</field>
                    </depends>
                </field>

                <field id="elasticsearch6_server_port" translate="label" type="text" sortOrder="72" showInDefault="1"
                       showInWebsite="0" showInStore="0">
                    <label>Elasticsearch Server Port</label>
                    <depends>
                        <field id="engine">elasticsearch6</field>
                    </depends>
                </field>

                <field id="elasticsearch6_index_prefix" translate="label" type="text" sortOrder="73" showInDefault="1"
                       showInWebsite="0" showInStore="0">
                    <label>Elasticsearch Index Prefix</label>
                    <depends>
                        <field id="engine">elasticsearch6</field>
                    </depends>
                </field>

                <field id="elasticsearch6_enable_auth" translate="label" type="select" sortOrder="74" showInDefault="1"
                       showInWebsite="0" showInStore="0">
                    <label>Enable Elasticsearch HTTP Auth</label>
                    <source_model>Magento\Config\Model\Config\Source\Yesno</source_model>
                    <depends>
                        <field id="engine">elasticsearch6</field>
                    </depends>
                </field>

                <field id="elasticsearch6_username" translate="label" type="text" sortOrder="75" showInDefault="1"
                       showInWebsite="0" showInStore="0">
                    <label>Elasticsearch HTTP Username</label>
                    <depends>
                        <field id="engine">elasticsearch6</field>
                        <field id="elasticsearch6_enable_auth">1</field>
                    </depends>
                </field>

                <field id="elasticsearch6_password" translate="label" type="text" sortOrder="76" showInDefault="1"
                       showInWebsite="0" showInStore="0">
                    <label>Elasticsearch HTTP Password</label>
                    <depends>
                        <field id="engine">elasticsearch6</field>
                        <field id="elasticsearch6_enable_auth">1</field>
                    </depends>
                </field>

                <field id="elasticsearch6_server_timeout" translate="label" type="text" sortOrder="77" showInDefault="1"
                       showInWebsite="0" showInStore="0">
                    <label>Elasticsearch Server Timeout</label>
                    <depends>
                        <field id="engine">elasticsearch6</field>
                    </depends>
                </field>

                <field id="elasticsearch6_test_connect_wizard" translate="button_label" sortOrder="78" showInDefault="1"
                       showInWebsite="0" showInStore="0">
                    <label/>
                    <button_label>Test Connection</button_label>
                    <frontend_model>Magento\Elasticsearch6\Block\Adminhtml\System\Config\TestConnection</frontend_model>
                    <depends>
                        <field id="engine">elasticsearch6</field>
                    </depends>
                </field>
            </group>
        </section>
    </system>
</config>