HypeWatch calls the YouTube Data API directly from your browser — there's no server in between, so it needs your own free key. It's stored only in this browser (localStorage), never sent anywhere but Google.
Open the Google Cloud Console and create a project.
APIs & Services → Library → search YouTube Data API v3 → Enable.
Click Restrict key → under API restrictions choose YouTube Data API v3 only, and under application restrictions add this site's domain (HTTP referrers) once it's deployed.