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

ButtonOverScrollView.xaml

<?xml version="1.0" encoding="utf-8"?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms" 
    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" 
    xmlns:local="clr-namespace:ScrollViewFormatting" 
    x:Class="ScrollViewFormatting.ScrollViewFormattingPage">

    <ContentPage.Padding>
        <OnPlatform x:TypeArguments="Thickness">
            <OnPlatform.iOS>0,20,0,0</OnPlatform.iOS>
        </OnPlatform>
    </ContentPage.Padding>
    
	<ContentPage.Content>

        <AbsoluteLayout>

            <RelativeLayout>

                <ScrollView BackgroundColor="Yellow"
                    RelativeLayout.XConstraint="0"
                    RelativeLayout.YConstraint="0"
                    RelativeLayout.WidthConstraint="{ConstraintExpression
            Type=RelativeToParent,Property=Width,Factor=1,Constant=0}"
                    RelativeLayout.HeightConstraint="{ConstraintExpression
            Type=RelativeToParent,Property=Height,Factor=1,Constant=0}"
                    >

                    <StackLayout Padding="20,20,20,20"
                    Spacing="20"
                    >

                    <Label Text="Dummy Text Here..."
                        TextColor="Black"
                        BackgroundColor="Lime" />

                    <Label Text="Dummy Text Here..."
                        TextColor="Black"
                        BackgroundColor="Lime" />

                    <Label Text="Dummy Text Here..."
                        TextColor="Black"
                        BackgroundColor="Lime" />
                        
                    <Label Text="Dummy Text Here..."
                        TextColor="Black"
                        BackgroundColor="Lime" />

                    <Label Text="Dummy Text Here..."
                        TextColor="Black"
                        BackgroundColor="Lime" />

                    <Label Text="Dummy Text Here..."
                        TextColor="Black"
                        BackgroundColor="Lime" />
                        
                    <Label Text="Dummy Text Here..."
                        TextColor="Black"
                        BackgroundColor="Lime" />

                    <Label Text="Dummy Text Here..."
                        TextColor="Black"
                        BackgroundColor="Lime" />

                    <Label Text="Dummy Text Here..."
                        TextColor="Black"
                        BackgroundColor="Lime" />  
                        
                    <Label Text="Dummy Text Here..."
                        TextColor="Black"
                        BackgroundColor="Lime" />

                    <Label Text="Dummy Text Here..."
                        TextColor="Black"
                        BackgroundColor="Lime" />

                    <Label Text="Dummy Text Here..."
                        TextColor="Black"
                        BackgroundColor="Lime" />
                        
                    <Label Text="Dummy Text Here..."
                        TextColor="Black"
                        BackgroundColor="Lime" />

                    <Label Text="Dummy Text Here..."
                        TextColor="Black"
                        BackgroundColor="Lime" />

                    <Label Text="Dummy Text Here..."
                        TextColor="Black"
                        BackgroundColor="Lime" />
                        
                    <Label Text="Dummy Text Here..."
                        TextColor="Black"
                        BackgroundColor="Lime" />

                    <Label Text="Dummy Text Here..."
                        TextColor="Black"
                        BackgroundColor="Lime" />

                    <Label Text="Dummy Text Here..."
                        TextColor="Black"
                        BackgroundColor="Lime" /> 

                    <Label Text="Dummy Text Here..."
                        TextColor="Black"
                        BackgroundColor="Lime" />

                    <Label Text="Dummy Text Here..."
                        TextColor="Black"
                        BackgroundColor="Lime" />

                    <Label Text="Dummy Text Here..."
                        TextColor="Black"
                        BackgroundColor="Lime" />
                        
                    <Label Text="Dummy Text Here..."
                        TextColor="Black"
                        BackgroundColor="Lime" />

                    <Label Text="Dummy Text Here..."
                        TextColor="Black"
                        BackgroundColor="Lime" />

                    <Label Text="Dummy Text Here..."
                        TextColor="Black"
                        BackgroundColor="Lime" />
                        
                    <Label Text="Dummy Text Here..."
                        TextColor="Black"
                        BackgroundColor="Lime" />

                    <Label Text="Dummy Text Here..."
                        TextColor="Black"
                        BackgroundColor="Lime" />

                    <Label Text="Dummy Text Here..."
                        TextColor="Black"
                        BackgroundColor="Lime" />  

                </StackLayout>
                    
                </ScrollView>
                
            </RelativeLayout>

            <Button Text="Press Me!"
                TextColor="White"
                BackgroundColor="Black"
                AbsoluteLayout.LayoutBounds="0.5, 0.95, 200, 50"
                AbsoluteLayout.LayoutFlags="PositionProportional"
            />
            
        </AbsoluteLayout>
        
    </ContentPage.Content>
    
</ContentPage>