Once you're in the notifications menu, you can see the Slack configuration settings.
Slack works by configuring webhooks that receive data, that in turn will be posted to a channel in your Slack. Here's how they explain it;
Incoming Webhooks are a type of integration that make it easy to post Slack messages from outside sources into your workspace. This is a great way to send data to your workspace in real-time.
Slack's official documentation explains in detail how to create your webhook.
On the Oh Dear! side, you only need to copy/paste your Slack webhook in our notification setting, determine which events should be sent and you're done. You determine the channel that it gets posted to, entirely from your Slack account.
For more details, see the official Slack documentation on webhooks.