Welcome code? – JapanTaxi Review

I am trying to register with this app, but it won’t let me get past the registration page. It’s asking for a welcome code, but what is that? I received the four-digit code, but that appears to be something different. The reviews are so good with this app, and seems like it’s just what we need for our upcoming trip to Japan. I’ll happily change my rating if I can figure out how to get registered...
Review by jentocco on JapanTaxi.