public class Client(string username, string password, int timeout = 60)

class Program
{
    private static int? Timeout => GetFromConfig("Timeout");

    static void Main(string[] args)
    {
        Client client = new Client(Username, Password, Timeout);
        // do things...
    }
}

static void Main(string[] args)
{
    if (Timeout == null)
    {
        Client client = new Client(Username, Password);
    }
    else
    {
        Client client = new Client(Username, Password, Timeout);
    }
}

public class Client(string username, string password, int? timeout)
{
    _timeout = timeout ?? 60;
}

public class Client(string username, string password, int? timeout = 60)
{
    _timeout = timeout ?? 60;
}

static void Main(string[] args)
{
    Client client = new Client(Username, Password, Timeout ?? usedefaultparam);
    // ...
}