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

{
  "id": "<string>",
  "external_id": "<string>",
  "email_address": "<string>",
  "phone_number": "<string>",
  "metadata": {},
  "unsubscribed": "future"
}