This page will send Spotify this redirect URI. It must be registered on your app, character for character:
developer.spotify.com/dashboard → your app → Settings → Edit → Redirect URIs → Add → Save
To play full songs this needs your own free Spotify app. Open developer.spotify.com/dashboard → Create app. Tick Web API and Web Playback SDK, and set the Redirect URI to exactly:
Then paste the Client ID here. It is not a secret.
Playback in this page needs Spotify Premium. Without it you can still browse, and every track links out to Spotify.