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>"
}
'

{}