curl --request POST \
  --url https://api.box.com/2.0/metadata_taxonomies/{namespace}/{taxonomy_key}/levels:append \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "displayName": "France",
  "description": "French Republic"
}
'

{
  "entries": [
    {
      "displayName": "Continent",
      "description": "Continent",
      "level": 1
    }
  ]
}