The MusixMatch API cannot be used anonymously. For this reason, you'll need to sign up for an account over at https://developer.musixmatch.com/signup.
Don't worry; this is free. The MusixMatch API's free tier will allow us to do enough for the purposes of this book. If you skip this, you won't be able to use the live API. Once you have filled in the registration form, you'll get a confirmation email.
Once your account has been confirmed, go to your applications administration page over at https://developer.musixmatch.com/admin/applications. There, you should see a default application listed. Click on the View button next to it to see its details.
On the details page, you should find your API key. Go ahead and add it to the src/api-key.ts file.