Open URL in browser from Message Button using Slack API

Unfortunately slack does not support opening urls from message buttons. You can monitor what slack is planning on releasing here though: https://trello.com/b/ZnTQyumQ/slack-platform-roadmap-for-developers :)

It appears Slack introduced this feature recently.

As documented in https://api.slack.com/docs/message-attachments#link_buttons

  "actions": [
    {
      "type": "button",
      "text": "Book flights 🛫",
      "url": "https://flights.example.com/book/r123456"
    }

It's possible to preview in Slack's interactive message builder