This tag allows you to send complex messages to slack. It leverages the chat.postMessage API.

To use this tag, you must have a Slack Application, you can do it this way:

1. Create a Slack Application on https://api.slack.com/apps?new_app=1
2. Chose "Incoming Webhooks" in the list of feature you want to install
3. Create a Webhook on the channel of your choice, and copy the link.
4. This link will be the value of Slack Webhook URL in GTM.

Then, you can create your complex message by using the Block Kit Builder: https://app.slack.com/block-kit-builder/. What is in the Payload on the right of this page is what is required in the Blocks parameter.

As you're on GTM, you can use GTM Variable to render your message dynamically (in the tag, set {{VariableName}} that will be replaced by the variable).

Here's an example you can send to slack when using the default Block Kit Builder Example: