I created an account on CloudFlare but I am unable to add my onrender.com site using the free CloudFlare plan. The page just keeps going back to the same step without any errors or other indication of what the problem is. Oddly, if I select one of their paid plans then it proceeds just fine.
You cannot add onrender.com, nor a specific subdomain of it, to your own Cloudflare account, you must use your own unique Custom Domain.