Automatically add a scene to your Build Settings when you create or save the scene. Scenes that are ignored will not re-prompt for the current session.

BuildSceneProcessor.cs

using UnityEngine;
using UnityEditor;
using System.Collections;
using System.Collections.Generic;

// docs: https://docs.unity3d.com/ScriptReference/AssetModificationProcessor.OnWillSaveAssets.html
public class BuildSceneProcessor : UnityEditor.AssetModificationProcessor
{
    private const string DIALOG_TITLE = "Add to Build Settings?";
    private const string DIALOG_MSG = "Add to build settings for inclusion in future builds?";
    private const string DIALOG_OK = "Yes";
    private const string DIALOG_NO = "Not now";

    private static List<string> ignorePaths = new List<string>();

    public static void OnWillCreateAsset(string path)
    {
        if (path.EndsWith(".unity.meta"))
            path = path.Substring(0, path.Length - 5);

        ProcessAssetsForScenes(new string[] { path });
    }

    public static string[] OnWillSaveAssets(string[] paths)
    {
        return ProcessAssetsForScenes(paths);
    }

    private static string[] ProcessAssetsForScenes(string[] paths)
    {
        string scenePath = string.Empty;

        foreach (string path in paths)
        {
            if (path.Contains(".unity"))
                scenePath = path;
        }

        // NOTE: add more logic here if you want to filter for certain paths.
        if (!string.IsNullOrEmpty(scenePath) && !ignorePaths.Contains(scenePath))
            AddSceneToBuildSettings(scenePath);

        // unity only saves the paths that you return here, so we always pass through everything we received.
        return paths;
    }

    private static void AddSceneToBuildSettings(string scenePath)
    {
        List<EditorBuildSettingsScene> scenes = new List<EditorBuildSettingsScene>(EditorBuildSettings.scenes);

        // only bother adding scenes we don't have already.
        foreach (EditorBuildSettingsScene scene in scenes)
        {
            if (scene.path == scenePath)
                return;
        }

        if (EditorUtility.DisplayDialog(DIALOG_TITLE, DIALOG_MSG, DIALOG_OK, DIALOG_NO))
        {
            EditorBuildSettingsScene newScene = new EditorBuildSettingsScene();
            newScene.path = scenePath;
            newScene.enabled = true;

            scenes.Add(newScene);
            EditorBuildSettings.scenes = scenes.ToArray();
        }
        else
        {
            ignorePaths.Add(scenePath);
        }
    }
}