Exactly, and that pre-configuration is fly ips allocate-v6 --private -a app-name. (Apparently, you don’t even need the --network flag, if you just want it to be accessible from the default network.)
Do this (or the corresponding GraphQL call) right after you create app-name, the target that you’re trying to forward things to.
Possibly I muddled the issue by suggesting that the Nginx proxy also be in a custom network!