user_ids not working on Send Push notifications

ionic-v1
1

I’m following this documentation: http://docs.ionic.io/docs/push-sending-push in order to get push notifications up and running.

However when I change:

   "tokens":[
    "token",
  ],

for

   "user_ids":[
    "user_id",
  ],

I receive a bad request error. Anyone else having this issue? Push notifications are working perfectly fine when using token but I’d like to use my user_id.