This app was mentioned in 3 comments, with an average of 2.33 upvotes
You can download Arianespace's iOS or Android apps.
https://itunes.apple.com/us/app/arianespace-hd/id582945211?mt=8
https://play.google.com/store/apps/details?id=com.paraweb.arianespace&hl=en
Yeah, I hear all the time about people saying they would watch rocket launches if they knew about them, but they always hear about it after the fact. Not sure if it is worth it to you to download another app, but Arianespace has there own app (Apple link , Android link). I'm not sure if it has launch alerts, but it may be something you want to check out.
Yes, here you go.
Also for future reference you can download the Arianespace apps on Google Play or the App store. They both give you notifications of the launch.