This adds a simple on/off checkbox to the Tools menu that allows you to toggle between starting with the current scene or the first scene in the build settings.

StartWithFirstScene.cs

#if UNITY_EDITOR
using UnityEditor;
using UnityEngine;

using UnityEditor.SceneManagement;

namespace Kamgam
{
	/// <summary>
	/// Start With First Scene<br />
	/// Sometimes when working on a scene in Unity you do not necessarily want the play mode
	/// to start with that scene (which is the default behavior). Instead, you may want it to
	/// start as it would in a build (aka the first scene is used).<br /><br />
	/// This adds a simple on/off checkbox to the Tools menu.
	/// </summary>
	/// <description>
	/// Based on an idea on this thread:
	/// http://forum.unity3d.com/threads/157502-Executing-first-scene-in-build-settings-when-pressing-play-button-in-editor
	/// </description>
	[InitializeOnLoad]
	public static class StartWithFirstScene
	{
		private const string NAME = "Tools/Start with first scene";

		static bool isEnabled
		{
			get
			{
				return EditorPrefs.GetBool(NAME, false);
			}
			set
			{
				EditorPrefs.SetBool(NAME, value);
				SceneListChanged();
			}
		}

		[MenuItem(NAME, false, 0)]
		static void LoadFirstScene()
		{
			isEnabled = !isEnabled;
		}

		[MenuItem(NAME, true, 0)]
		static bool ValidateLoadFirstScene()
		{
			Menu.SetChecked(NAME, isEnabled);
			return true;
		}

		static StartWithFirstScene()
		{
			SceneListChanged();

			EditorBuildSettings.sceneListChanged += SceneListChanged;
		}

		static void SceneListChanged()
		{
			if (!isEnabled)
			{
				EditorSceneManager.playModeStartScene = default;
				return;
			}
			//Ensure at least one build scene exist.
			if (EditorBuildSettings.scenes.Length == 0)
			{
				Debug.Log("No Scenes in Build Settings");
				isEnabled = false;
				return;
			}
			//Reference the first scene
			SceneAsset theScene = AssetDatabase.LoadAssetAtPath<SceneAsset>(EditorBuildSettings.scenes[0].path);
			// Set Play Mode scene to first scene defined in build settings.
			EditorSceneManager.playModeStartScene = theScene;
		}
	}
}
#endif