fixture.cs

public class Fixture<TStartup> : WebApplicationFactory<TStartup> where TStartup : class
{
    public HttpClientInterceptorOptions InterceptorOptions { get; } =
        new HttpClientInterceptorOptions {ThrowOnMissingRegistration = true};

    protected override void ConfigureWebHost(IWebHostBuilder builder)
    {
        builder.ConfigureServices(s
            => s.AddSingleton<IHttpMessageHandlerBuilderFilter, InterceptionFilter>(
                _ => new InterceptionFilter(InterceptorOptions)));

        base.ConfigureWebHost(builder);
    }

    private sealed class InterceptionFilter : IHttpMessageHandlerBuilderFilter
    {
        private readonly HttpClientInterceptorOptions _options;

        internal InterceptionFilter(HttpClientInterceptorOptions options)
            => _options = options;

        public Action<HttpMessageHandlerBuilder> Configure(Action<HttpMessageHandlerBuilder> next)
            => builder =>
            {
                next(builder);
                builder.AdditionalHandlers.Add(_options.CreateHttpMessageHandler());
            };
    }
}

Which namespase should I use for HttpClientInterceptorOptions ?