CategoryService.php

class CategoryService {

        /**
         * Build category selector
         *
         * @param array $config array (passed by reference) which contains the current information about the current field being rendered
         * @param \TYPO3\CMS\Backend\Form\FormEngine $formEngine a reference to the parent object (an instance of the t3lib_TCEforms class).
         *
         * @return string
         */
        public function categorySelector($config, \TYPO3\CMS\Backend\Form\FormEngine &$formEngine) {

                $categories = $GLOBALS['TYPO3_DB']->exec_SELECT_mm_query('sys_category.uid, sys_category.title, sys_category_record_mm.locked', 'sys_category', 'sys_category_record_mm', $config['table'], 'AND uid_foreign = ' . $config['row']['uid'], '', 'sys_category.title');

                while ($currentCategories[] = $GLOBALS['TYPO3_DB']->sql_fetch_row($categories)) {
                }
                array_pop($currentCategories);
                $javscript = '
                        <script type="text/javascript" src="/typo3conf/ext/bolius_products/Resources/Public/Scripts/Suggest.js" ></script>
                ';

                $formEngine->additionalCode_pre['tx_boliusproducts_categoryselector'] = $javscript;

ext_tables.php

                                'type' => 'user',
                                'userFunc' => 'Bolius\\BoliusProducts\\Service\\CategoryService->categorySelector'