You can try to use following approach: Override OnResume in the activity of the root view model, then navigate to the target view model(for example: AboutViewModel).

I created a simple project by using Blank project template. By default, SimpleViewModel is set as the RootViewModel (see override OnStart in AppService.cs file of Core project). I added a layout in the Android project and named it as About.axml. I also add AboutActivity and AboutViewModel.

In SimpleActivity class, override OnResume and navigate to the AboutViewModel by using following snippet code.

protected override void OnResume()
{
    base.OnResume();

    this.ViewModel.NavigationService.Navigate(new NavigationTarget(typeof (AboutViewModel), NavigationMode.Modal));
}

Hope this help.

I put the code into my RootViewModel but I get:

protected override void OnResume() is marked as an override but no suitable method found to override.

And
If I put this into AppServices.cs in the core, I get Core/Infrastructure/AppService.cs(18,18): Error CS0120: An object reference is required to access non-static member `Intersoft.Crosslight.ViewModels.ViewModelBase.NavigationService' (CS0120) (IncroudRadio.Core)

protected override void OnResume()
{
    base.OnResume();

    HomeViewModel.NavigationService.Navigate(new NavigationTarget(typeof (AdViewModel), NavigationMode.Modal));
}

I uploaded my simple project in here. Please have the project tested on your end and let me know whether it helps or not.

For your information, I'm using Crosslight 2 Update 5 assembly files in my local end.

Ok this kinda works for Android, it keeps popping up the AdViewModel over and over again which I can fix.

What about iOS?

        public override void DidEnterBackground (UIApplication application)
        {
    
        }

I'm unable to put the code to the ViewModel in there within the AppDelegate.cs

What about iOS?

You can try to override ViewDidAppear in MainViewController.cs and add following snippet code.

public override void ViewDidAppear(bool animated)
{
    base.ViewDidAppear(animated);

    this.ViewModel.NavigationService.Navigate(new NavigationTarget(typeof(AboutViewModel), NavigationMode.Modal));
}

Hope this help.