curl --request POST \
  --url https://api.revring.ai/v1/voices/design \
  --header 'Content-Type: application/json' \
  --header 'x-api-key: <api-key>' \
  --data '
{
  "instruction": "<string>",
  "n": 2
}
'

{
  "data": {
    "candidates": [
      {
        "index": 123,
        "audioBase64": "<string>",
        "durationMs": 123,
        "sampleRate": 123
      }
    ]
  }
}