To start, have your agent go into their Slack settings and grab the slack email that get's created as noted below in the Slack instructions
Now take that copied email and were going to create a new target and trigger in Zendesk:
Here is a link about notifying external targets, but to get you going quickly... go to Settings->Extensions
Next screen we will add a target:
Select Email target:
Add the target similar to below:
Now go to your triggers and create a trigger similar to below or any type of trigger you want. Make sure to select notify target and choose the target you just created.
Now in the agents slackbot channel they'll get a message similar to this:
Since it's an email, you should be able to also add additional html formatting for links and more!