The 403 generally occurs when the CORS url is not configured correctly.
You can configure the CORS URL for your APP Key using your developer account here: https://console.temasys.io
Please see below a screenshot of CORS configuration to be able to run on cdpn.io and localhost. Note that CORS is configured by default to run on all sub-domains.
Other articles you may be interested in: This key is not authorized for use on this domain| Can I use wildcards to denote subdomains in the CORS URL?