How do I configure an AWS network load balancer with an A record in Cloudflare
You create a CNAME record referencing the assigned hostname of the balancer as the target.

CNAME records are not valid at the apex of a zone (i.e. the "naked" domain, without www or any other prefix) but Cloudflare allows you to create one there -- sort of -- they actually allow you to believe that you're creating one there, and then their service automatically transforms it into one or more valid A-records, dynamically.

This feature is enabled by default at the apex.

https://blog.cloudflare.com/introducing-cname-flattening-rfc-compliant-cnames-at-a-domains-root/