In the main Xaml, use the ItemsControl balice and for the Xaml linker, you must define your project using UserControl instead of Window
<ItemsControl x:Name="Plugins" x:FieldModifier="private" Height="auto">
In your code you just need to write this:
Plugins.Items.Add(UserControl);
Perhaps you can learn something about MEF to download all your Xaml
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