WPF: Set Startup Window.

You can set your Startup page by setting StartupUri="MainWindow.xaml"  in App.xaml file. 
<Application x:Class="MyApp.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             StartupUri="MyWindow.xaml">
    <Application.Resources>
   
    </Application.Resources>

</Application>