json_schema: {
      "additionalProperties": false,
      "description": "A join_group GTM server-side event.",
      "properties": {
          "group_id": {
              "description": "The ID of the group.",
              "maxLength": 4096,
              "type": [
                  "string",
                  "null"
              ]
          }
      },
      "self": {
          "format": "jsonschema",
          "name": "join_group",
          "vendor": "com.google.tag-manager.server-side",
          "version": "1-0-0"
      },
      "type": "object"
  }