By default you will get App.xaml as follows:

<Application x:Class="Login_WPF.App" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
StartupUri="MainWindow.xaml">
 <Application.Resources></Application.Resources>
</Application>

In the above App.xaml default window is "Main Window". if you want to open "Registration" window first then change the StartUpUrl property of the application in App.xaml like as follows:
<Application x:Class="Login_WPF.App"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
StartupUri="Registration.xaml">
<Application.Resources></Application.Resources>
</Application>