Signing in is optional. It stores your runs in the cloud so they follow you across devices, and lets you use a shared server-side API key instead of your own.
Paste any of: @handle, a channel ID or URL, a playlist URL, a video URL
(watch?v=, youtu.be, /shorts/), or a bare video ID.
With your own key, everything runs in your browser and the key is saved on this device only.