Magento 2 REST API Customer Custom Attribute

The Magento 2 REST API Documentation explains a way to set custom_attributes on a customer when updating or creating it. http://devdocs.magento.com/swagger/index_20.html#/

Unfortunately I couldn't get this to work...

My POST and PUT JSON request data is :

{
    "customer": {
        "custom_attributes": [
            {
                "attribute_code": "firstname",
                "value": "TEST"
            }
        ],
        "email": "someone@exaxmple.com",
        "extension_attributes": [],
        "firstname": "Someone",
        "gender": null,
        "lastname": "Else",
        "middlename": null,
        "taxvat": null,
        "website_id": "1"
    }
}

The Customer is created but the Firstname is not "TEST". Is there anyone who had the same problem and fixed it? Please let me know how.

My best guess is that, since Firstname is an existing Out-Of-The-Box attribute - the OOTB attribute name-value mapping assignment will take precedence.

Could you try again, with a unique custom attribute name (i.e. something that doesn't clash with OOTB attribute names)

You will need to DEFINE a custom-customer-attribute before you can use the M2 API to perform operations on that custom-customer-attribute.

This StackExchange thread - https://magento.stackexchange.com/questions/88245/magento2-create-a-customer-custom-attribute - has additional information on how to go about setting up a custom-customer-attribute.