at the bottom (or just about anywhere) in your HTML.
The other, codepen specific way is to go into your Settings -> JS -> Add External Scripts/Pens and add the url there, in one of the slots.
Either of those should make a green “hamburger” appear in the upper left of your page. Press on that, select your project, and run the tests. It will tell you if you didn’t pass.
In addition, when a test fails click the red button to see which test(s) are failing and text to help you correct the issue.
Be sure and read more than just the first line of the failing message. The ability to read and comprehend error messages is a skill you’ll need to acquire as a developer. Ask questions on what you don’t understand.