Blazor Service does not get injected with bUnit

I have a problem with my .NET 8 Blazor bUnit Tests, where I try to get the services injected with the Services.AddSingleton<>() command, because I have an inject @inject CartService CartService, that causes an error, if I try to run the tests without (see bUnit Website as reference).

But it still is not working and throwing an error, that Services does not have a method called AddSingleton(). Does anybody have an idea, what I can do here?

public void ItemsAreShowedOnOverview() {    Services.AddSingleton<CartService>(new CartService());    using var ctx = new TestContext();    var cutOverview = ctx.RenderComponent<Overview>();    cutOverview.Find("p").MarkupMatches("<p>Taktiles Messgerät</p>");}

I tried to run without the Services.AddSingleton<>() but it will not work an throw an error, that the service is not registered.