Electron Fiddle Gist

index.html

<!-- Empty -->

main.js

// Modules to control application life and create native browser window
const {app, BrowserWindow} = require('electron')

// Keep a global reference of the window object, if you don't, the window will
// be closed automatically when the JavaScript object is garbage collected.
let mainWindow

function createWindow () {
  // Create the browser window.
  mainWindow = new BrowserWindow({
    width: 800,
    height: 600,
    webPreferences: {
      nodeIntegration: false,
      contextIsolation: true,
      nativeWindowOpen: true,
      preload: require.resolve('./renderer.js')
    }
  })

  // and load the index.html of the app.
  mainWindow.loadURL('https://app.slack.com')

  // Open the DevTools.
  mainWindow.webContents.openDevTools()

  // Emitted when the window is closed.
  mainWindow.on('closed', function () {
    // Dereference the window object, usually you would store windows
    // in an array if your app supports multi windows, this is the time
    // when you should delete the corresponding element.
    mainWindow = null
  })
}

// This method will be called when Electron has finished
// initialization and is ready to create browser windows.
// Some APIs can only be used after this event occurs.
app.on('ready', createWindow)

// Quit when all windows are closed.
app.on('window-all-closed', function () {
  app.quit()
})

renderer.js

let picker

window.addEventListener('message', (evt) => {
  if (evt.origin !== 'https://www.dropbox.com') return
  if (!evt.data) return

  console.log(evt.data)

  const parsedData = JSON.parse(evt.data)
  switch (parsedData.method) {
    case 'files_selected':
    case 'close_dialog':
      picker.close()
  }
})

window.addEventListener('DOMContentLoaded', () => {
  picker = window.open('https://www.dropbox.com/chooser?origin=https%3A%2F%2Fapp.slack.com&app_key=hz986p4r827h6kt&link_type=preview&trigger=js&multiselect=true&extensions=&folderselect=false&iframe=false&version=2')
})