Tasks are executed after the job is triggered and are the main building blocks of a job. You can string together as many tasks as you want.

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All tasks

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postMessage

Post a message to a channel. Official Slack Docs
example.ts
// Send a Slack message using the io.slack.postMessage function
await io.slack.postMessage("post message", {
  // Specify the target channel by providing its ID
  channel: "<your-channel-id>",
  // Set the text content of the message
  text: "<your-slack-message>",
});

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Using the underlying client

If we don’t have the task you need listed above, you can use io.slack.runTask() function, which lets you do anything that’s possible with the official Slack Node SDK. More information here. Example usage: 
client.defineJob({
  id: "send-slack-message",
  name: "Send a Slack message",
  version: "1.0.0",
  trigger: eventTrigger({
    name: "send.slack.message",
    schema: z.object({
      message: z.string,
    }),
  }),
  integrations: {
    slack,
  },
  run: async (payload, io, ctx) => {
    // Use the Slack integration to create a task using the "post-message" cacheKey
    const message = await io.slack.postMessage("post-message", {
      channel: process.env.SLACK_CHANNEL_ID,
      message: payload.message,
    });

    // Use the Slack integration's runTask method to add a reaction to the message
    await io.slack.runTask("add-reaction", async (client) => {
      // client here is an authenticated instance of the Slack SDK
      await client.reactions.add({
        channel: process.env.SLACK_CHANNEL_ID,
        name: "thumbsup",
        timestamp: message.ts,
      });
    });
  },
});

​
How to post rich messages to Slack

Use their Block kit builder, and then use the blocks property to send the message.
linearIssuesDailySlackAlert.ts
...
await io.slack.postMessage("post message", {
      channel: process.env.SLACK_CHANNEL_ID!,
      // Include text for notifications and blocks to get a rich Slack message in the channel
      text: `You have ${inProgressIssues.nodes.length} 'In Progress' issues in Linear!`,
      // Create rich Slack messages with the Block Kit builder https://app.slack.com/block-kit-builder/
      blocks: inProgressIssues.nodes.flatMap((issue) => [
        {
          type: "section",
          text: {
            type: "mrkdwn",
            text: `⏳ *${issue.title}*`,
          },
          accessory: {
            type: "button",
            text: {
              type: "plain_text",
              text: "View issue",
              emoji: true,
            },
            value: "click_me_123",
            url: issue.url,
            action_id: "button-action",
          },
        },
        {
          type: "divider",
        },
      ]),
    });
  },
});