If you want to use application first you need to change API key in the applicaton.json file. If you don't have an API key You need to go Spotify Developer Platform, create an app get get your own client id and secret.
Then update the applicaton.json file like this

  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft.AspNetCore": "Warning"
    }
  },
  "Spotify": {
    "ClientId": "YOUR-CLIENT-ID",
    "ClientSecret": "YOUR-CLIENT-SECRET"
  },
  "AllowedHosts": "*"
}

And one last step open \Spotify-Asp\SpotifyASP directory in your command prompt and write

dotnet watch

Now you're good to go! If you find any bugs or you have any ideas you can create a pull request anytime.