Thailand Government Open Data.
Open Government, Thailand API is a free Government API that requires apiKey authentication, is served over HTTPS and has unconfirmed CORS support.

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Yes. The Open Government, Thailand API is listed in the Government category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
Yes. The Open Government, Thailand API uses apiKey authentication, so you need to register for credentials before making requests.
CORS support for the Open Government, Thailand API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The Open Government, Thailand API is served over HTTPS, so it's safe to use in production.