title: How to show a modal window in windows 10 universal app?
body: <p>When I use Mail univesal app in windows 10, when i add an account (setting->accounts->add account), it seems popup a modal window to choose an account. I try to use MessageDialog, but i can't put any custom content into it.</p>
  
  <p><strong>EDIT</strong> : this is the screenshot
  <a href="https://i.stack.imgur.com/0K6RS.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/0K6RS.png" alt="screenshot"></a></p>
  
  <p>Is someone knows how to implement it or there is some api can do it?</p>
  
  <p><strong>Note:</strong> When this window open, you even can't Minimize/Maximize/Close the main Window. So, it is definitely a modal window.</p>