curl --request POST \
  --url https://billing.funnelfox.com/{org_id}/v1/search_users \
  --header 'Content-Type: application/json' \
  --header 'ff-secret-key: <api-key>' \
  --data '
{
  "email": "<string>"
}
'

{
  "users": [
    {
      "external_id": "<string>"
    }
  ]
}