curl -X POST
    -H 'accept: application/json'
    -H 'content-type: application/json'
'https://api.mercadopago.com/v1/subscriptions?access_token=ACCESS_TOKEN'
    -d '{
            "plan_id": "PLAN_ID",
            "payer": {
                    "id": "CUSTOMER_ID" <--- ??
            }
    }'