EditorSceneSettingsExtension.cs

using System.Collections.Generic;
using UnityEditor;
using UnityEditor.SceneManagement;

static class EditorSceneSettingsExtension
{

    [SettingsProvider]
    public static SettingsProvider CreateStartSceneSettingsProvider()
    {
        var provider = new SettingsProvider("Project/Editor/PlayModeStart", SettingsScope.Project)
        {
            label = "Playmode Start",
            guiHandler = searchContext =>
            {
                EditorSceneManager.playModeStartScene = EditorGUILayout.ObjectField("Startup Scene", EditorSceneManager.playModeStartScene, typeof(SceneAsset), false) as SceneAsset;
            },
            keywords = new HashSet<string> {"Playmode", "Scene", "Startup"}
        };
        return provider;
    }
}