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
Create a new resource in Retool, and select "Slack" as the type. Enter your webhook URL. Press "Save" when you're done.
Resources > Add > Slack
You can now select your newly-created Slack resource from the Resource dropdown when creating queries in your Retool apps. We currently only support plaintext messages.
When you "Preview" a Slack query, it will send a message to your Slack workspace.