Unity Editor - Always Load at a Particular Scene

 If you work with multiple scenes, and use them additively you might like to start at a particular one when you press the play button in the editor.

Here's a class to help you out:

using UnityEditor;
using UnityEditor.SceneManagement;
using UnityEngine; 

[InitializeOnLoad]
public class SetStartSceneForEditor
{

    static SetStartSceneForEditor()
    {
        if (Application.isEditor)
        {
            EditorSceneManager.playModeStartScene =
                AssetDatabase.LoadAssetAtPath<SceneAsset>("Assets/Scenes/Container.unity");
        }
    }
}