After some initial investigation, it seems that at least maybe dropping a custom script into the directory
/boot/config/plugins/dynamix/notifications/agents/
would work based on the code in
/usr/local/emhttp/plugins/dynamix/scripts/notify
but I haven't tested this. However, a UI way of doing this would be preferred.
EDIT:
Creating a script
/boot/config/plugins/dynamix/notifications/agents/ntfy
worked (as a temp solution). Here's my example for ntfy.sh
#!/bin/bash
curl -u "user:pass" \
-H "Title: ${SUBJECT}" \
-H "Tags: unRAID" \
-d "Event: ${EVENT}
Subject: ${SUBJECT}
Description: ${DESCRIPTION}
Importance: ${IMPORTANCE}
Time: $(date '+%Y-%m-%d %H:%M:%S')
${CONTENT}" \
https://ntfy.domain.tld/unRAID