Fetch orders from backend to Intercom

Hi,

I am trying to use Custom Objects to show the orders of a users that message us, either through email or whatsapp. I created the custom object Order with the relevant attributes. I then created an external reference in People and Conversations, referencing Order via a one to many relationship. 

Then I created a webhook that triggers when a conversation is created or a reply is given. The webhook calls a service that posts to /custom_object_instances/Order

We post an external_id and custom_attributes according to https://developers.intercom.com/docs/references/rest-api/api.intercom.io/custom-object-instances/createcustomobjectinstances 

-d '{
    "external_id": "123",
    "external_created_at": 1392036272,
    "external_updated_at": 1392036272,
    "custom_attributes": {
      "order_number": "ORDER-12345",
      "total_amount": 99.99
    }

Now the documentation does not say anything about how to link the Order to People.
Fin says, you need to put a customer property inside custom_attributes, fine we do that. But the Order reference in the sidebar doesn’t show anything. We put the customer outside custom_attributes, it still does not work. 

What are we missing?