I went to Origin Certificates -> Create Certificate and saved both files: cert.pem and cert.key
And downloaded Cloudflare Origin CA — RSA Root to cf_ca.pem
An origin cert is signed by Cloudflare for use as a trusted cert between Cloudflare and your origin. That it returns an error as being self signed when accessed directly by an end user is expected.
curl -Ik https://localhost:8899 will ignore the self signed nature of the cert. However port 8999 isn’t a port Cloudflare proxies.
Cloudflare does not offer certificates to be installed on 3rd party servers directly from our CA partners, you would need to obtain a cert from a CA directly