Told you, Dagger.

Program.cs

    private void AddSongToQueue(string trackUri)
    {
        if (string.IsNullOrEmpty(accessToken))
        {
            GD.Print("Access token not available. Authenticate.");
            return;
        }

        var queueUrl = $"https://api.spotify.com/v1/me/player/queue?uri={Uri.EscapeDataString(trackUri)}";
        var headers = new[]
        {
            $"Authorization: Bearer {accessToken}",
            "Content-Length: 0"
        };

        httpRequest.Request(queueUrl, headers, HttpClient.Method.Post);
    }