You can use the selection box to define the application resource through which the Designer is to be started. All the resources from this file will be loaded. Mostly this is the resource file App.xaml from the main application. If you have a modular application, you can even select an App.xaml from your solution.
If you have a special case and do not have an App.xaml, or you want to further customize the designer to your needs, you can even create your own XAML file.
This file must be called "FastWpf.xaml", it should be an "Application" XAML similar to that of a standard application. This FastWPF.xaml can also have a code-behind file FastWpf.xaml.cs if desired, this way you can even customize the designer startup process if desired.
The selection of this selection box is automatically saved on a project-by-project basis. This means once configured, FastWPF remembers this setting for further use.