Creating new related records with Ember Data

I'm trying to create two new related records at once, and I'm wondering the best way to go about this from Ember Data.

I have a User and a Reminder. I need to create the user first, then create the user's reminder.

I could write my server code to handle a payload like

reminder: {
  text: 'something',
},
user: {
  email: 'bob@acme.com'
}

but, how would I get Ember Data to send that payload? Write a custom serializer? On which model, user or reminder?

Alternatively, should I make the two POST requests separately, first the user then the reminder? Is this more idiomatic?