, put CSS in the styles file, JS in the file with the script. Most often these files are in some folder assets and are called style.css and app.js respectively.
And the html code wherever you want. And this applies not only to Wordpress sites. You also didn't specify where you are going to insert this piece of code. In the theme files, or maybe directly in the article editor?
For a less abstract answer, I need to see the file structure of your project, get the information where you will include the html code, see the content of the head tag of your theme.
If you don't want to figure it out, you can put this plugin https://ru.wordpress.org/plugins/custom-css-js/
. It will allow you to write CSS and JS code without interfering with the theme files.
But the html code must be added by hand.
Although for this case I recommend not to put anything, it is too simple a task.