This is what you will get:
You need:
The incoming webhook URL as given by flock.com. You can use the same on all your Netdata servers (or you can have multiple if you like - your decision).
Get them here: https://admin.flock.com/webhooks
Set them in /etc/netdata/health_alarm_notify.conf (to edit it on your system run /etc/netdata/edit-config health_alarm_notify.conf), like this:
###############################################################################
# sending flock notifications
# enable/disable sending pushover notifications
SEND_FLOCK="YES"
# Login to flock.com and create an incoming webhook.
# You need only one for all your Netdata servers.
# Without it, Netdata cannot send flock notifications.
FLOCK_WEBHOOK_URL="https://api.flock.com/hooks/sendMessage/XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"
# if a role recipient is not configured, no notification will be sent
DEFAULT_RECIPIENT_FLOCK="alarms"