Slack Notification Help

CMK version: Checkmk Cloud Edition 2.3.0p28
I am testing things in CMK to see if I want to use this long-term. Currently I cannot get slack notifications to work at all. Is there anything I am missing in the configuration? Do I need to configure anything on our checkmk linux server?

This is what I have done so far in CMK Notifications:
Notification Method…

Slack or Mattermost

Create notification with the following parameters

Webhook-URL

Webhook URL
https-----

Disable SSL certificate verification
URL prefix for links to Checkmk
HTTP proxy

Connect without proxy

Connect directly to the destination instead of using a proxy.
Notification Bulking…

I have it alerting one user and nearly every alert checked.

In slack I have created the app/webhook and assigned it to the appropriate channel.

I have tried using fake check results and manually turning the server on and off but I don’t see any messages coming through to the slack channel. Am I missing something?