curl -i -X PUT "https://api.box.com/2.0/metadata_templates/enterprise/securityClassification-6VMVochwUWo/schema" \
     -H "authorization: Bearer <ACCESS_TOKEN>" \
     -H "content-type: application/json-patch+json" \
     -d '[{
       "op": "addEnumOption",
       "fieldKey": "Box__Security__Classification__Key",
       "data": {
         "key": "Sensitive",
         "staticConfig":{
          "classification": {
            "classificationDefinition": "Sensitive information that must not be shared.",
            "colorID": 4
            }
         }
       }
     }]'

{
  "id": "58063d82-4128-7b43-bba9-92f706befcdf",
  "type": "metadata_template",
  "scope": "enterprise_123456",
  "templateKey": "securityClassification-6VMVochwUWo",
  "displayName": "Classification",
  "fields": [
    {
      "id": "822227e0-47a5-921b-88a8-494760b2e6d2",
      "type": "enum",
      "key": "Box__Security__Classification__Key",
      "displayName": "Classification",
      "options": [
        {
          "id": "46aea176-3483-4431-856c-6b5b13d1cc50",
          "key": "Sensitive",
          "staticConfig": {
            "classification": {
              "classificationDefinition": "Sensitive information",
              "colorID": 4
            }
          }
        }
      ],
      "hidden": false
    }
  ],
  "hidden": false,
  "copyInstanceOnItemCopy": true
}