Through an automation tool
Zapier, Make or any tool that can receive a webhook and post to Slack.
Create a webhook trigger
In your automation tool, start a new workflow that begins with an incoming webhook, and copy the HTTPS address it gives you.
Add it in SiteMind
In your SiteMind dashboard, open Webhooks, add the address, and choose the lead.captured event.
Send a test
Use the test button on the webhook so your tool receives a sample event and learns its fields.
Post to Slack
Add a Slack step that sends a message to your channel, using the name, email, phone and message fields from the event.