First, you can try to add SKStoreReviewController.RequestReview(Window.WindowScene) in the FinishedLaunching method of your xxx.ios->AppDelegate.cs; start the project to see if it is correct.

If the above method goes wrong, using DependencyService is the right way to go.

Here is the interface code:

 public interface MyInterface
{
     void RequestReview();
}

Here is the implementation method of the interface in ios:

[assembly: Dependency(typeof(MyInterfaceImpl))]
namespace App19.iOS
{
    public class MyInterfaceImpl : MyInterface
    {
        public void RequestReview()
        {
           var myv = UIDevice.CurrentDevice.CheckSystemVersion(14, 0);
            if (myv)
            {
                UIWindow window = UIApplication.SharedApplication.Delegate.GetWindow();
            SKStoreReviewController.RequestReview(window.WindowScene);
            }
            else 
            {
                SKStoreReviewController.RequestReview();
            }
        }
    }
}

You can call it in the OnStart method inside APP.xaml.cs:

 protected override void OnStart()
    {
        if (Device.RuntimePlatform == Device.iOS)
        {
            DependencyService.Get<IReviewService>().RequestReview();
        }
    }

I might have a solution:

using StoreKit;
using UIKit;

public void RequestReview(){
    var scene = UIApplication.SharedApplication.KeyWindow.WindowScene;

    if (scene is null)
    {
        //handle the scene being null here
        return;
    }

    SKStoreReviewController.RequestReview(scene);
}

I've found multiple ways of getting the current scene and they both worked when I tested them:

var scene = UIApplication.SharedApplication.KeyWindow.WindowScene;

var alsoScene = UIApplication.SharedApplication.Delegate.GetWindow()?.WindowScene;

You can read more about the difference between the two in this issue

The complete solution where you check the iOS version in order to call the right thing might look like this

using StoreKit;
using UIKit;

public void RequestReview()
{
    var isIos14OrAbove = UIDevice.CurrentDevice.CheckSystemVersion(14, 0);
    if (isIos14OrAbove)
    {
        var scene = UIApplication.SharedApplication.KeyWindow.WindowScene;

        if (scene is null)
        {
            //handle the scene being null here
            return;
        }

        SKStoreReviewController.RequestReview(scene);
    }
    else
    {
        SKStoreReviewController.RequestReview();
    }
}

Hope this helps.