In the cryengine 5 manual in documentation under using C# UI > Creating basic UI. The line of code Engine.Shutdown(); gives an error that says 'Engine' does not contain a definition for Shutdown(). So it should be cryengine.Engine.Engine3D.Shutdown();
http://docs.cryengine.com/display/CEPRO ... a+Basic+UI

The Engine.Shutdown() method is added in CRYENGINE 5.4, which is currently in preview. The documentation has been updated to reflect 5.4 now instead of 5.3. To make this work in 5.3 you can copy the content of the Shutdown method from Github.