message.json

{
  "attachments": [
    {
      "fallback": "users",
      "callback_id": "b75e3599-b12b-412a-88ff-65d0cad93864",
      "actions": [
        {
          "name": "foo",
          "type": "select",
          "text": "assign to",
          "data_source": "users",
          "selected_options": [
            {
              "value": "U0BTHL3JT"
            }
          ]
        }
      ]
    }
  ],
  "unfurl_links": false,
  "channel": "#staging",
  "as_user": true
}