base64 decode

base64_decode('base64');const b64string = /* whatever */;
const buf = Buffer.from(b64string, 'base64');
class Program
{
    static void Main(string[] args)
    {
        var account = new ExternalAccount() { Name = "Someone" };
        string json = JsonConvert.SerializeObject(account);
        string base64EncodedExternalAccount = Convert.ToBase64String(Encoding.UTF8.GetBytes(json));
        byte[] byteArray = Convert.FromBase64String(base64EncodedExternalAccount);

        string jsonBack = Encoding.UTF8.GetString(byteArray);
        var accountBack = JsonConvert.DeserializeObject<ExternalAccount>(jsonBack);
        Console.WriteLine(accountBack.Name);
        Console.ReadLine();
    }
}

[Serializable]
public class ExternalAccount
{
    public string Name { get; set; }
}