$categories = new Select('categories', Constants::$FirmCategories, [
            'id' => 'categories',
            'name' => 'categories[]',
            'multiple' => true,
            'placeholder' => 'Multiple Select'
        ]);
$categories->setLabel('<i class="fa fa-star-o red"></i> Categories);
$this->add($categories);

<div class="form-group">
    {{ form.label("categories",['class': 'col-sm-3 control-label no-padding-right']) }}
    <div class="col-sm-4">
        {{ form.render("categories",['class': 'form-control select2']) }}
    </div>
</div>

    public function afterFetch()
    {
        $this->categories = explode(',', $this->categories);

        $this->categories = array_combine($this->categories, $this->categories);

        $this->introduction = htmlspecialchars($this->introduction);
    }

        if (!$this->request->isPost()) {

            $firm = Firms::findFirst(.....);

            if (!$firm) {
                return $this->response->redirect('/firms/index');
            }

            //var_dump($firm->categories);
            $this->view->form = new FirmsForm($firm);
            return;
        }

        /**
         * Generate the OPTION tags based on an array
         *
         * @param array data
         * @param mixed value
         * @param string closeOption
         */
        private static function _optionsFromArray(data, value, closeOption)
        {
                var strValue, strOptionValue, code, optionValue, optionText, escaped;

                let code = "";

                for optionValue, optionText in data {

                        let escaped = htmlspecialchars(optionValue);

                        if typeof optionText == "array" {
                                let code .= "\t<optgroup label=\"" . escaped . "\">" . PHP_EOL . self::_optionsFromArray(optionText, value, closeOption) . "\t</optgroup>" . PHP_EOL;
                        } else {
                                if typeof value == "array" {
                                        if in_array(optionValue, value) {
                                                let code .= "\t<option selected=\"selected\" value=\"" . escaped . "\">" . optionText . closeOption;
                                        } else {
                                                let code .= "\t<option value=\"" . escaped . "\">" . optionText . closeOption;
                                        }
                                } else {
                                        let strOptionValue = (string) optionValue,
                                                strValue = (string) value;
                                        if strOptionValue === strValue {
                                                let code .= "\t<option selected=\"selected\" value=\"" . escaped . "\">" . optionText . closeOption;
                                        } else {
                                                let code .= "\t<option value=\"" . escaped . "\">" . optionText . closeOption;
                                        }
                                }
                        }
                }

                return code;
        }