How can I prove the ownership of my free domain at infinityfree.net

[KangJL]

You need a custom domain.
Free subdomain will not do.

See below for more ino

Where do I get free domains?

You can try using EU.org or PP.UA (the latter one if you have a credit card even with a few cents on it and a phone number to use for verification, though the credit card is used only once for verification and then what’s free will remain free) as a valid alternative to Freenom (and Dot TK because Dot TK and Freenom are the same) that also supports Cloudflare and other things that Freenom with its domains supported.