Xamarin Forms : How to make button appear at the bottom of a page

This worked for me.

<StackLayout BackgroundColor="#2D3033"> 
                <Button Clicked ="Button_Clicked"
                            Text="Login"  
                            BackgroundColor="#007F00"
                            BorderColor="#004C00"
                            BorderWidth="1"
                            TextColor="white"
                        HorizontalOptions="CenterAndExpand"
                        VerticalOptions="EndAndExpand"
                    />
        </StackLayout>

<ContentPage.Content>
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto" />
            <RowDefinition Height="*" />
        </Grid.RowDefinitions>

        <StackLayout Grid.Row="0">
        </StackLayout>

        <StackLayout Grid.Row="1" VerticalOptions="End">
            <controls:STButton Text="Yeni Görev Ekle" />
        </StackLayout>
   </Grid>
</ContentPage.Content>

%100 works ;)

With a Grid is simple just do this:

<?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:shared_forms" x:Class="shared_forms.shared_formsPage">
    <Grid>
        <Label Text="Enter A Code" VerticalOptions="Center" HorizontalOptions="Center" />
        <Button Text="Reveal Code" x:Name="RevealCode" HorizontalOptions="CenterAndExpand" VerticalOptions="End" />
    </Grid>
</ContentPage>

With StackLayout:

<?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:shared_forms" x:Class="shared_forms.shared_formsPage">
    <StackLayout VerticalOptions="FillAndExpand" HorizontalOptions="FillAndExpand">
        <StackLayout Orientation="Horizontal" VerticalOptions="Start">
            <!-- top controls -->
            <Label Text="Enter A Code" VerticalOptions="Center" HorizontalOptions="CenterAndExpand" />
        </StackLayout>
        <StackLayout VerticalOptions="CenterAndExpand">
            <!-- middle controls -->
        </StackLayout>
        <StackLayout Orientation="Horizontal" VerticalOptions="End">
            <!-- bottom controls -->
            <Button Text="Reveal Code" x:Name="RevealCode" HorizontalOptions="CenterAndExpand" />
        </StackLayout>
    </StackLayout>
</ContentPage>

Result: