Not sure why no one has pointed you to the actual product Cloudflare have for API security - Cloudflare API Gateway (and API Shield).
You can kinda-sorta-not-really fudge control with a combination of Access Policy (or exclusion rules for that) and Firewall Rules, or even tack on Access control via JWT etc if you want though.
Withuot any of those just consider it having been made 'public' to the internet at large and secure accordingly.