How to access the angular6 json schema form library core meta schema

Question

I am using angular6-json-schema-form library for the first time and created a example.json file with schema and data section

How to reference the angular6-json-schema-form library referenced draft version of core meta schema definations from example.json

"schema": {
  "$schema": "http://json-schema.org/draft-07/schema#",
  "$id": "http://someurl/example.json",
  "type": "object",
  "properties": {
    "id": {
      "type": "number"
    },
    "formName": {
      "type": "string",
      "maxlength": "$ref": "http://json-schema.org/draft-07/schema/definitions/nonNegativeInteger       
    }
  }
},
"data": {
  "id": "1",
  "formName": "Application Form"
}