Ok, thanks for your reply, I really appreciate your support : )
I found a solution, but it's more like a workaround I haven't thought of before:
Just create a second window which runs monogame. One for GUI, one for "Game". In my case this is perfectly fine but for real games this is of course no solution.
Solution is also super simple (detailed explanation if a beginner might stumble upon it):
1. Create WPF project.
2. Add Reference to Monogame.Framework (C:\Program Files (x86)\MonoGame\v3.0\Assemblies\Windows\MonoGame.Framework.dll)
3. Create a new Monogame for Windows project and copy the Content folder and the Game1.cs class in your project.
4. Add folowing code in MainWindow.xaml.cs and add a Button in MainWindow.xaml with Click="StartEngine_Click"
public partial class MainWindow : Window
public Game1 monoEngine = new Game1();
private void StartEngine_Click(object sender, RoutedEventArgs e)