curl --request POST \
  --url https://api.pocketflows.com/sync/businesses/{business_id}/customers/batch \
  --header 'Authorization: Basic <encoded-value>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "customers": [
    {
      "id": "<string>",
      "external_id": "<string>",
      "phone_number": "<string>",
      "email_address": "<string>",
      "metadata": {}
    }
  ]
}
'

{
  "customers": [
    {
      "id": "<string>",
      "external_id": "<string>",
      "email_address": "<string>",
      "phone_number": "<string>",
      "metadata": {},
      "unsubscribed": "future"
    }
  ],
  "errors": [
    "<unknown>"
  ]
}