public class FooClient
    private IHttpClientFactory _clientFactory;
    private FooClientConfig _config;        
    public FooClient(IHttpClientFactory clientFactory, FooClientConfig config) 
        _clientFactory = clientFactory;
        _config = config;           
}

public class Startup : FunctionsStartup
    public void ConfigureServices(IServiceCollection services)
        services.AddHttpClient();
        var container = new Container();
        container.RegisterInstance<FooClientConfig>(new FooClientConfig() { ApiKey = Configuration.GetValue<string>("FooApiKey") });
        container.Register<FooClient>();
        services.AddSimpleInjector(container);
}

public class FooCommandHandler 
    private readonly FooClient _fooClient;