Cloudflare Origin Certificate for HTTPS Backend Protocol

Anonymous
I'm trying to configure my application gateway backend settings to use HTTPS protocol with a Origin CA certificate generated by Cloudflare. I followed the steps here to create the certificate, download as PEM and saved as CER. When I attempt to save the backend settings, it is rejected as invalid.
The Cloudflare forums directed me to this approach, but I'm hoping someone with experience on the Azure side can tell me if this is correct set up or if I'm possibly missing a step?