How can I call a method in a view model from the App.xaml.cs or an external background service project in the same solution

Question

I am trying to call a method in a View Model from a background service that is hosting Cortana. I am unsure how to link up either the background service or App.xaml.cs to call a method in any view model whilst adhering to MVVM. Any suggestions would be appreciated! Thanks

I was thinking in the App.xaml.cs I could set up events that get called and invoke methods in the viewmodel by registering with that event