A few month ago, I wrote an article about the Mist Guest logger script I created. You can check it out here: https://www.networkjon.fr/blog/juniper-mist-guest-logger-script-for-the-built-in-captive-portal/
I created an alternative version of this script. Instead of dealing with Websockets, the new version uses Webhook. The script is therefore simpler and more stable. Here it is:
https://github.com/jrambeau/mist-guest-logger-webhook
Enjoy and let me know if the script helped you and what we could improve!