CropImage.cs

private async Task CropImage()
{
    // Create destination file and obtain sharing token
    var file = await ApplicationData.Current.LocalFolder.CreateFileAsync("Cropped.jpg", CreationCollisionOption.ReplaceExisting);
    var token = SharedStorageAccessManager.AddFile(file);
    
    // Specify the app to launch using LaunchUriForResults
    var options = new LauncherOptions();
    options.TargetApplicationPackageFamilyName = "Microsoft.Windows.Photos_8wekyb3d8bbwe";
    
    // Specify all the parameters
    var parameters = new ValueSet();
    parameters.Add("CropWidthPixels", 500);
    parameters.Add("CropHeightPixels", 500);
    parameters.Add("EllipticalCrop", true);
    parameters.Add("ShowCamera", false);
    parameters.Add("DestinationToken", token);
    
    // Launches the app and wait for results
    var result = await Launcher.LaunchUriForResultsAsync(new Uri("microsoft.windows.photos.crop:"), options, parameters);
    // Check if the user has really cropped the picture
    if (result.Status == LaunchUriStatus.Success && result.Result != null)
    {
        // Loads the picture
        var stream = await file.OpenReadAsync();
        var bitmap = new BitmapImage();
        await bitmap.SetSourceAsync(stream);
        
        // Set the picture as Source of an Image control
        Preview.Source = bitmap;
    }
}

Here is how to do this with your own image directly (versus letting the user pick one in the Image Gallery). Just add the token as an input parameter as well (and write something to file before, of course):
parameters.Add("InputToken", token);

I noticed that CropWidthPixels and CropHeightPixels don't work on Desktop. It only works on Mobile. Will this be fixed?

I noticed that CropWidthPixels and CropHeightPixels are not consistently respected, it worked on Desktop for me with Photo App version16.511.8780.0, but not in 17.313.10010.0

Thanks! Just what I was looking for.
But I can't take pictures in portrait, the app flips them heads to bottom.
I have noticed the issue repeats itself in the people app.

I would also like to us LaunchUriForResultsAsync to launch my other app. My problem is that it only launches another app in 500x500 window size. I need something bigger than that. Is it possible to make it full screen or maybe any size bigger than 500?