index.js

const functions = require('firebase-functions');
const { App } = require('@slack/bolt');

const app = new App({
  token: process.env.SLACK_BOT_TOKEN,
  signingSecret: process.env.SLACK_SIGNING_SECRET
});

// Listens to incoming messages that contain "hello"
app.message('hello', ({ message, say }) => {
  // say() sends a message to the channel where the event was triggered
  say({
    blocks: [
      {
        "type": "section",
        "text": {
          "type": "mrkdwn",
          "text": `Hey there <@${message.user}>!`
        },
        "accessory": {
          "type": "button",
          "text": {
            "type": "plain_text",
            "text": "Click Me"
          },
          "action_id": "button_click"
        }
      }
    ]
  });
});

app.action('button_click', ({ body, ack, say }) => {
  // Acknowledge the action
  ack();
  say(`<@${body.user.id}> clicked the button`);
});

exports.app = functions.https.onRequest(app);

package.json

{
  "name": "first-bolt-app",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "author": "",
  "license": "ISC",
  "dependencies": {
    "@slack/bolt": "^1.4.1",
    "firebase-functions": "^3.3.0"
  },
  }
}