You'll first need to set up an "Incoming Webhook" in Slack: https://my.slack.com/apps/new/A0F7XDUAZ-incoming-webhooks.
Once you select a channel and create the webhook, you'll be presented with a setup screen containing details about your webhook and how to use it. Make a note of the "Webhook URL", which will be in the following format:
Slack > Custom Integrations > Incoming Webhooks > New Configuration
Enter your Webhook URL in the respective field. Click 'Save' to add Slack to Jet Admin.