MyContentPage.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

using Xamarin.Forms;

namespace LoadingScreen
{
    class MyContentPage : ContentPage
    {
        public MyContentPage ()
        {
            StackLayout loadingView = new StackLayout
            {
                Orientation = StackOrientation.Horizontal,
                BackgroundColor = Color.Gray,
                Children =
                {
                    new ActivityIndicator
                    {
                        Color = Color.White,
                        IsRunning = true,
                        VerticalOptions = LayoutOptions.Center,
                        WidthRequest = 20,
                        HeightRequest = 20
                    },
                    new Label
                    {
                        TextColor = Color.White,
                        Text = "Loading...",
                        VerticalOptions = LayoutOptions.Center
                    }
                }
            };

            AbsoluteLayout layout = new AbsoluteLayout
            {
                HorizontalOptions = LayoutOptions.FillAndExpand,
                VerticalOptions = LayoutOptions.FillAndExpand,
                Children =
                {
                    {   // this is the "existing content" placeholder
                        new BoxView { Color = Color.Accent },
                        new Rectangle(0,0,1,1),
                        AbsoluteLayoutFlags.All
                    },
                    {
                        loadingView,
                        new Rectangle(
                            0.5, 0.5,
                            AbsoluteLayout.AutoSize, AbsoluteLayout.AutoSize),
                        AbsoluteLayoutFlags.PositionProportional
                    }
                }
            };

            this.Content = layout;
        }
    }
}