Editor script to make Play button always start a main scene in Unity 3D (works only with 2017.2 or +)

PlayFromScene.cs

using UnityEditor;
using UnityEditor.SceneManagement;
using UnityEngine;

namespace SmartEditors
{
    [InitializeOnLoad]
    public class PlayFromScene : Editor
    {
        [MenuItem("Tools/Editor/Play From Scene/Set Main Scene", false, 0)]
        public static void SetScene()
        {
            SceneAsset playModeStartScene = AssetDatabase.LoadAssetAtPath<SceneAsset>(EditorSceneManager.GetActiveScene().path);
            if (playModeStartScene != null)
                EditorSceneManager.playModeStartScene = playModeStartScene;
            else
                Debug.Log("Scene not saved!");
        }

        [MenuItem("Tools/Editor/Play From Scene/Unset Main Scene", false, 1)]
        public static void UnsetScene()
        {
            EditorSceneManager.playModeStartScene = null;
        }

        [MenuItem("Tools/Editor/Play From Scene/Unset Main Scene", true, 1)]
        public static bool UnsetSceneValidate()
        {
            return EditorSceneManager.playModeStartScene != null;
        }
    }
}

Actually, I tested it and it does work indeed :)
The API change was added in 2017.1

https://docs.unity3d.com/2017.1/Documentation/ScriptReference/SceneManagement.EditorSceneManager-playModeStartScene.html