interface ILoginHandler
{
    Task LoginAsync();
}

public class DefaultLoginHandler : ILoginHandler
{
    private username, password;

    public DefaultLoginHandler(string username, string password){ this.username = username; this.password = password; }

    public async Task LoginAsync() => login logic
}

public class Site : ILoginHandler
{
    ILoginHandler loginHandler;

    public Site(ILoginHandler loginHandler){this.loginHandler = loginHandler; }

    public async Task LoginAsync() => await loginHandler.LoginAsync();
}