Spotify.NET/SpotifyAPI.Docs/versioned_docs/version-5.1.1/auth/client_credentials.md

id	title
client_credentials	Client Credentials

With this approach, you make a POST Request with a base64 encoded string (consists of ClientId + ClientSecret). You will directly get the token (Without a local HTTP Server), but it will expire and can't be refreshed. If you want to use it securely, you would need to do it all server-side. NOTE: You will only be able to query non-user-related information e.g search for a Track.

More info: here

CredentialsAuth auth = new CredentialsAuth(_clientId, _secretId);
Token token = await auth.GetToken();
SpotifyWebAPI api = new SpotifyWebAPI()
{
  TokenType = token.TokenType,
  AccessToken = token.AccessToken
};