C# Code that demonstrates how you can add a button over a scroll view for a Xamarin app

ButtonOverScrollView.xaml.cs

using Xamarin.Forms;
using System;


namespace ScrollViewFormatting
{
    public partial class ScrollViewFormattingPage : ContentPage
    {
        public ScrollViewFormattingPage()
        {
            InitializeComponent();

            //Define UI
            AbsoluteLayout absoluteLayout = new AbsoluteLayout();
            RelativeLayout relativeLayout = new RelativeLayout();
            ScrollView scrollView = new ScrollView();
            BoxView scrollBox = new BoxView();
            StackLayout stackView = new StackLayout();
            Button myButton = new Button();

            //Format UI
            scrollBox.BackgroundColor = Color.Yellow;
            scrollBox.HeightRequest = 2000;
            scrollBox.HorizontalOptions = LayoutOptions.Fill;

            stackView.Padding = new Thickness(10, 10, 10, 10);
            stackView.Spacing = 20;

            //Add alot of labels to the stack view (so the stackview's height is greater than the screens)
            for (int i = 1; i < 30; i++) {
                stackView.Children.Add(CreateLabel());
            }

            scrollView.Content = stackView;

            //Add scrollview to relative layout
            relativeLayout.Children.Add(scrollView,
                xConstraint: Constraint.Constant(0),
                yConstraint: Constraint.Constant(0),
                widthConstraint: Constraint.RelativeToParent((parent) => { return parent.Width; }),
                heightConstraint: Constraint.RelativeToParent((parent) => { return parent.Height; }));

            //Format button
            myButton.Text = "Press Me!";
            myButton.BackgroundColor = Color.Black;
            myButton.TextColor = Color.White;
            myButton.HorizontalOptions = LayoutOptions.CenterAndExpand;
            myButton.VerticalOptions = LayoutOptions.CenterAndExpand;
            myButton.WidthRequest = 200;
            AbsoluteLayout.SetLayoutBounds(myButton, new Rectangle(0.5, 0.95, 200, 50));
            AbsoluteLayout.SetLayoutFlags(myButton, AbsoluteLayoutFlags.PositionProportional);

            //Add views to layout & set as our view
            absoluteLayout.Children.Add(relativeLayout);
            absoluteLayout.Children.Add(myButton);

            Content = absoluteLayout;
        }

        //Helper method to create alot of labels
        public Label CreateLabel() {

            Label localLabel = new Label();
            localLabel.Text = "Sample Text...";
            localLabel.TextColor = Color.Black;
            localLabel.BackgroundColor = Color.LimeGreen;

            return localLabel;
        }
    }
}