Yes, ntfy. Alright figured it out. Here are the steps:

1) Install ntfy from F-Droid
2) If you have already done so, and there are entries that are of the form ntfy.sh/xGH... ➡️ Delete them
3) In ntfy app > Settings > Default Server: change to your self hosted url (e.g https://ntfy.mydomain.tld)
4) In Amethyst, log in with Amber, and follow the poop up steps. The process will make the ntfy app to create a new subscription, but this time to your server (not ntfy.sh)