Fdbbackup to backblaze b2 / cloudflare r2?

I have a basic fdbserver + backup_agent running. I can do local backups via fdbbackup file:/// without a problem.

I am trying to backup to either cloudflare r2 or backblaze b2, but despite reading the docs of fdbbackup --help, I am having trouble constructing the right URL.

Does anyone have fdbbackup to cloudflare r2 / backblaze b2 working? If so, can you, after replacing api_key with XXX and secret_key with YYY, post your script?

I am running fdb 7.1.x.