Bulk create contact

curl --request POST \
  --url https://app.usesend.com/api/v1/contactBooks/{contactBookId}/contacts/bulk \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
[
  {
    "email": "<string>",
    "firstName": "<string>",
    "lastName": "<string>",
    "properties": {},
    "subscribed": true
  }
]
'

{
  "message": "<string>",
  "count": 123
}