curl --request POST \
  --url https://v1.api.flux-os.com/contracts/{id}/parties/{contract_party_id}/users \
  --header 'Content-Type: application/json' \
  --header 'x-api-key: <api-key>' \
  --header 'x-user-id: <api-key>' \
  --data '
{
  "user": "<unknown>"
}
'

{
  "data": {
    "party": "<unknown>",
    "id": "<string>",
    "user": {
      "first_name": "<string>",
      "last_name": "<string>",
      "id": "<string>",
      "roles": "<unknown>"
    },
    "created_at": "2023-11-07T05:31:56Z",
    "updated_at": "2023-11-07T05:31:56Z"
  }
}