Slack Dialog with Google Apps Script

dialog.gs

var SLACK_ACCESS_TOKEN = PropertiesService.getScriptProperties().getProperty('SLACK_ACCESS_TOKEN');

function doGet(e) {  
  return ContentService.createTextOutput("I am alive");
}

function doPost(e) {    
  var params = e.parameter;
  var token = params.token;
  var text = params.text;
  var trigger_id = params.trigger_id;
  var slackUrl = "https://slack.com/api/dialog.open";
  
  var dialog = {
    "token": SLACK_ACCESS_TOKEN, 
    "trigger_id": trigger_id,
    "dialog": JSON.stringify({
      "callback_id": "ryde-46e2b0",
      "title": "Submit a Slack-Update",
      "submit_label": "Update",
      "elements": [
        {
          "type": "text",
          "label": "Yesterday",
          "name": "yesterday",
          "placeholder": "What did you finish yesterday?"
        },
        {
          "type": "text",
          "label": "Today",
          "name": "today",
          "placeholder": "What will you work on today?"
        },
        {
          "type": "text",
          "label": "Blockers",
          "name": "blockers",
          "value": "None"
        }
      ]
    })
  }
  var options = {
    'method' : 'post',
    'payload' : dialog,
  }; 
  UrlFetchApp.fetch(slackUrl, options);
  return ContentService.createTextOutput("");
}