Reference base in different project/dll

namespace NameOfApp
{
    public partial class App : Application
    {
        public App()
        {
            InitializeComponent();
            MainPage = new AppShell();
        }

        protected override Window CreateWindow(IActivationState activationState)
        {
            //this should be handled by the other DLL
             var window = base.CreateWindow(activationState);
            //

           return DifferentDLL.SetCustomSize(base);
        }
}
}