How do I use both the --save-storage feature to save auth while using a custom setup?

Question

On this page, Playwright recommends to use the --save-storage option with Codegen in order to save the auth state. It also recommends just below to run a JS script instead of calling Codegen in CLI if we want to use a custom setup. Now, I need to do both, i.e. save the auth state AND run a custom setup. But I can't find a way to do that in the documentation.