Point subdomain via CNAME?

[ChrisPAR]

Unfortunately, this cannot be done. InfinityFree requires the nameservers to be changed before adding a domain to the hosting platform.

You can follow this guide, but it will still require changing the nameservers on your apex domain temporarily:

Hosting only a custom subdomain with InfinityFree

Hello! This guide will explain how you can host a subdomain of a domain name you own with InfinityFree without hosting the main domain too. Here is the example I will use in the guide. Note: I will be using terms that you may not recognize. Please read the first section of this article on domain names if you get confused. You own the domain example.com and want to add the subdomain blog.example.com to InfinityFree. However, you do not want example.com to be hosted with InfinityFree. Here is…

You could also try seeing if setting NS records for your subdomain to InfinityFree’s nameservers allows you to add it.