Mine is a similar answer to Dennis, with a bit more re-use.
I created a control template in App.xaml
<ControlTemplate x:Key="ActivityIndicatorTemplate">
<Grid>
<ContentPresenter />
<StackLayout Style="{StaticResource BlockingPanel}"
IsVisible="{TemplateBinding BindingContext.IsBusy}">
<ActivityIndicator Style="{StaticResource ActivityIndicatorStyle}"
IsVisible="{TemplateBinding BindingContext.IsBusy}"
IsRunning="{TemplateBinding BindingContext.IsBusy}" />
</StackLayout>
</Grid>
</ControlTemplate>
Here are the styles
<Style x:Key="BlockingPanel" TargetType="StackLayout">
<Setter Property="BackgroundColor" Value="{StaticResource BlockingColor}" />
<Setter Property="HorizontalOptions" Value="FillAndExpand" />
<Setter Property="VerticalOptions" Value="FillAndExpand" />
</Style>
<Style x:Key="ActivityIndicatorStyle" TargetType="ActivityIndicator">
<Setter Property="Color" Value="White" />
<Setter Property="HorizontalOptions" Value="CenterAndExpand" />
<Setter Property="VerticalOptions" Value="CenterAndExpand" />
</Style>
<Color x:Key="BlockingColor">
<x:Arguments>
<x:Double>0</x:Double>
<x:Double>0</x:Double>
<x:Double>0</x:Double>
<x:Double>0.75</x:Double>
</x:Arguments>
</Color>
Then you can use it on any page like this
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
x:Class="Incident.Pages.LoginPage"
ControlTemplate="{StaticResource ActivityIndicatorTemplate}"
Title="Login">
...
</ContentPage>