Xamarin.Forms MVVM: How to pass parameter to detail viewmodel?

We're trying to follow an MVVM pattern as supported by Xamarin.Forms, but have had some trouble figuring out the best way to respect MVVM conventions while navigating from a master page to a detail page.

Using this as an example:
https://developer.xamarin.com/guides/cross-platform/xamarin-forms/user-interface/xaml-basics/data_bindings_to_mvvm/

Forms supports assigning a view the type of its viewmodel, so the viewmodel is created automatically:

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
         xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
         xmlns:local="WidgetProjectNamespace"
         x:Class="WidgetProjectNamespace.MasterPage"
         Title="Master Page">
    <ContentPage.BindingContext>
      <local:MasterViewModel />
    </ContentPage.BindingContext>
    <StackLayout>
        <!-- A list of widgets goes here -->
    </StackLayout>
</ContentPage>

Let's say I have a master page with a simple list of items. The MasterViewModel will know how to retrieve the list of items and display them:

public class MasterViewModel
{
    ...
    public void Load()
    {
        var widgets= widgetRepository.Fetch();
        WidgetListItemSource = widgets;
    }
}

Tapping on an item will navigate to a detail page for that item. When I handle the click event, I need to navigate to a detail page by pushing it onto the stack:

Item.OnTapped += (s, e) => Navigation.PushAsync(new DetailPage());

That detail page can also have its viewmodel created automatically:

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
         xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
         xmlns:local="WidgetProjectNamespace"
         x:Class="WidgetProjectNamespace.DetailPage"
         Title="Detail Page">
    <ContentPage.BindingContext>
      <local:DetailViewModel />
    </ContentPage.BindingContext>
    <!-- Some content describing the widget's details goes here -->
</ContentPage>

But how will the DetailViewModel know what model it needs to display?

The only way to do this seems to be to tell the page directly:

Item.OnTapped += (s, e) => Navigation.PushAsync(new DetailPage(itemModel));

But this seems backwards. The viewmodel should tell the page what to display, not the other way around.

Am I missing something? Is there a way in vanilla Xamarin.Forms to navigate to a detail page and respect MVVM principles?