I am trying to create or upsert a Sales Order via the API using an external ID for a lookup. https://www.zoho.com/crm/developer/docs/api/v6/records-api-ext-id-overview.html

I use an external ID for the Contact Lookup:
"Contact_Name":{"FastSpring_Account_ID":"l6NPd8ygTm-qR5Vrfgru8rg"}, but the API returns an error required field (id) not found,

1. {
     "response": {
       "status": 400,
       "body": {
         "data": [
           {
             "code": "MANDATORY_NOT_FOUND",
             "details": {
               "api_name": "id",
               "json_path": "$.data[0].Contact_Name.id"
             },
             "message": "required field not found",
             "status": "error"
           }
         ]
       }
     },
     "request": {}
   }

Why? The FastSpring_Account_ID is properly set as an external ID at contact level. 

I've tried to Pass the following headers, but with no success so far:

1. X-EXTERNAL: Sales_Orders.Contact_Name.Contact.FastSpring_Account_ID
   
2. X-EXTERNAL: Sales_Orders.Contact_Name.FastSpring_Account_ID
3. X-EXTERNAL: Contact.Contact_Name.FastSpring_Account_ID
   
4. X-EXTERNAL: Contact.FastSpring_Account_ID

I want to use the external ID to avoid an extra API call returning the actual ID of the contact.