curl --request POST \
  --url https://realtime-api.voiceflow.com/v1/stable/variable \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "name": "<string>",
  "color": "<string>",
  "description": "<string>",
  "defaultValue": "<string>"
}
'

{
  "variable": {
    "id": "<string>",
    "name": "<string>",
    "color": "<string>",
    "isSystem": true,
    "createdAt": "2023-11-07T05:31:56Z",
    "updatedAt": "2023-11-07T05:31:56Z",
    "description": "<string>",
    "defaultValue": "<string>"
  }
}