curl --request POST \
  --url https://api.sendrealm.com/contacts/{contact_id}/audiences/{audience_id} \
  --header 'Authorization: Bearer <token>'

{
  "status": 123,
  "data": {
    "id": "3c90c3cc-0d44-4b50-8888-8dd25736052a",
    "contact_id": "3c90c3cc-0d44-4b50-8888-8dd25736052a",
    "audience_id": "3c90c3cc-0d44-4b50-8888-8dd25736052a",
    "is_subscribed": true,
    "created_at": "2023-11-07T05:31:56Z",
    "updated_at": "2023-11-07T05:31:56Z"
  },
  "error": null,
  "metadata": null
}