title: How to get a user's wechat open id from a web app?
body: <p>I have a reservations web app and we are thinking of sending a message to the user via wechat after he makes a reservation.</p>
  
  <p>I looked around the wechat documentation and see that we will need the user's openid but I have no clue how to obtain this.</p>
  
  <p>Do I have to program a "login via wechat" button?</p>