question

Given that the name of your InkCanvas is InkCanvas

<InkCanvas  x:Name="InkCanvas" />

To save at each stroke you can

• use the StrokesCollected and StrokesErased events
  

• use Application.Suspending event

Using StrokesCollected and StrokesErased this can be your Page.Loaded event handler:

private async void OnLoaded(object sender, RoutedEventArgs args)
{
    InkCanvas.InkPresenter.StrokesCollected += async (presenter, eventArgs) => { await Save(); };
    InkCanvas.InkPresenter.StrokesErased += async (presenter, eventArgs) => { await Save(); };
    await Load();
}

Using Application.Suspending this can be your Page.Loaded event handler:

private async void OnLoaded(object sender, RoutedEventArgs args)
{
    Application.Current.Suspending += async (o, suspendingEventArgs) =>
    {
        SuspendingDeferral deferral = suspendingEventArgs.SuspendingOperation.GetDeferral();
        await Save();
        deferral.Complete();
    };
    await Load();
}

the Save and Load methods:

private async Task Save()
{
    StorageFile file = await ApplicationData.Current.LocalFolder.CreateFileAsync("strokes.ink",
            CreationCollisionOption.ReplaceExisting);
    using (IRandomAccessStream stream = await file.OpenAsync(FileAccessMode.ReadWrite))
    {
        await InkCanvas.InkPresenter.StrokeContainer.SaveAsync(stream);
        await stream.FlushAsync();
    }
}

private async Task Load()
{
    IStorageItem storageItem = await ApplicationData.Current.LocalFolder.TryGetItemAsync("strokes.ink");
    if (!(storageItem is StorageFile storageFile)) return;
    using (IRandomAccessStream stream = await storageFile.OpenAsync(FileAccessMode.Read))
    {
        await InkCanvas.InkPresenter.StrokeContainer.LoadAsync(stream);
    }
}

I'm sure that file access in the Save method can be improved, anyway this works as a basic example.