.rf.gd is a free subdomain extension provided by InfinityFree, but due to hosting and Cloudflare limitations, you cannot use Cloudflare on a free subdomain. However, you can use it on paid custom domains (e.g., example.com). But take note that you need to buy the domain.
If you are looking for free domains (the ones that are on PSL), you can go with eu.org or pp.ua: