elasticpress-autosuggest-endpoint.php

 /**
 * Load Elasticseach PHP 7 Client via composer
 * Updated for ElasticPress 3.3
 */
require 'vendor/autoload.php';

/**
 * Init Elasticsearch PHP Client
 */
use Elasticsearch\ClientBuilder;
use ElasticPress\Indexables as Indexables;
use ElasticPress\Utils;

/**
 * Register Elasticpress Autosuggest Endpoint
 *
 * This is the endpoint you have to specify in the admin
 * like this: http(s)://domain.com/wp-json/elasticpress/autosuggest/
 */
add_action( 'rest_api_init', function() {
	register_rest_route( 'elasticpress', '/autosuggest/', [
		'methods' => \WP_REST_Server::CREATABLE,
		'callback' => 'ep_autosuggest',
	] );
} );

/**
 * Elasticpress Autosuggest Endpoint Callback
 *
 * gets host and index name dynamically. Otherwise,
 * if not specified, host would default to localhost:9200
 * and index name would default to 'index'
 *
 * @param \WP_REST_Request $data
 * @return array|callable
 */
function ep_autosuggest( \WP_REST_Request $data ){

	$client = ClientBuilder::create();
	$client->setHosts( [ElasticPress\Utils\get_host()] ); // get host dynamically
	$client = $client->build();	

	$json_params = $data->get_json_params();

	$params = [
		'index' => Indexables::factory()->get( 'post' )->get_index_name(), // get index dynamically
		'body' => $json_params
	];

	$response = $client->search( $params );

	return $response;
}

/*
* Exclude attachments from autosuggest results
*/
function ep_autosuggest_options($epas_options){

	unset($epas_options['mimeTypes']);
	$epas_options['mimeTypes'] = [];

	return $epas_options;		
}
add_filter( 'ep_autosuggest_options', 'ep_autosuggest_options', 20, 1 ); 

function filter_term_suggest_post_status($post_status){

	unset($post_status);
	$post_status = [ 'publish' ];

	return $post_status;

}
add_filter( 'ep_term_suggest_post_status', 'filter_term_suggest_post_status', 10, 1 );

Where does this file go?

@MikeLx You should turn this file into a WordPress plugin, or use it as one of the files inside a WordPress plugin. Check out this https://developer.wordpress.org/plugins/plugin-basics/header-requirements/