Connecting the plugin class to functions.php

I want to modify the plugin by adding a method to the class function.php how do I do this?

It didn't work out that way

вот так сделал поправил эту строку
require_once $_SERVER['DOCUMENT_ROOT']."/wp-content/wp-recall/add-on/top-user-by-rating/class/Tub_Rating.php";


add_shortcode('top_user_rating2', 'rcl_top_user_by_rating2');
function rcl_top_user_by_rating2($atts) {
  //Параметры шорткода
  $atts = shortcode_atts(array(
    'templates' => 'block',
    'sorting' => '1',
    'limit' => '50',
    'day' => '30',
    'exclude' => '',
    'type' => '',
    'period' => '',
    'paged' => '',
    'role' => '',
    'cache_time' => '86400'
  ), $atts);

тут объявил экземпляр класа
$tubrating = new Tub_Rating($atts);

тут присвоил функцию 
 $tubrating->getTopUserHtml2 = function() {


  return $result;
  };
  return $tubrating->getTopUserHtml2();

}

Connecting the plugin class

require(":/wp-content/wp-recall/add-on/class/Tub_Rating.php"); 

So I can't connect

require(): Failed opening required '/wp-content/wp-recall/add-on/class/Tub_Rating.php' (include_path='.:/opt/php71/share/pear') 

So too

require_once $_SERVER['DOCUMENT_ROOT']."/wp-content/wp-recall/add-on/class/Tub_Rating.php"