SlackCodeSyntaxHighlighter.js

// ==UserScript==
// @name         Slack Code Syntax Highlighter
// @namespace    M_Ishikawa
// @version      0.1
// @description  Click code block, highlight it.
// @author       Masayuki Ishikawa
// @match        https://*.slack.com/*
// @grant        none
// ==/UserScript==

/**
 * Thanks for highlight.js.
 */
(function() {
    'use strict';

    // 'monokai-sublime', 'xcode', 'darcula', 'solarized-dark', 'solarized-light', 'github', 'github-gist', 'xcode', 'androidstudio', ... and more.
    // @see https://highlightjs.org/static/demo/
    let theme = 'monokai';

    $('body').prepend(`<script src="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/9.12.0/highlight.min.js"></script>`);
    $('body').prepend($('<style/>').load('https://cdnjs.cloudflare.com/ajax/libs/highlight.js/9.12.0/styles/' + theme + '.min.css'));

    $(window).on('click', function(e) {
        if ($(e.target).prop('tagName') !== 'PRE' || ! $(e.target).hasClass('c-mrkdwn__pre') || $(e.target).attr('data-highlighted')) {
            return;
        }

        $(e.target).wrapInner('<code/>');
        $(e.target).attr('data-highlighted', true);
        hljs.highlightBlock(e.target);
    });
})();

How can I use this in Slack application? (MacOS)