Put your custom notification scripts in local/share/check_mk/notifications
as described in https://docs.checkmk.com/master/en/notifications.html and
ensure that they are executable (chmod +x).

All executable files will be considered targets for notifications.
If you want your scripts to have user readable aliasses then put those into
a comment into the second line, e.g.:

#!/usr/bin/foolang
# Notify via Loudspeaker
...

You script retrieves all necessary information about the actual notification
in environment variables beginning with NOTIFY_.