Using flake-parts from a non-parts flake

Trying to use services-flake, which is utilizing flake-parts. There’s a very basic example of how to use it without flake-parts, but I can’t figure out anything beyond that example. I.e., how can I reference the process-compose without a config variable, e.g., services-flake/example/simple/flake.nix at 88283a73a9c028d00307ca268229aef8e680b89c · juspay/services-flake · GitHub?

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