Web authentication workflow for apps without a front-end canister

Do you know how I can test if my process was successful in using the custom key? I mean what should I check to verify it is working an using the key I provided correctly?

You can deploy a whoami canister. You’ll know you’re successful if you can log in with II, check your principal, log out, and then log in a second time, getting the same principal back.

Thank you all for your help and insights. For anyone who might be looking for the end solution, based on what everyone said here, I created this proxy page:

functionland/Web-Identity-Providers: This is a proxy that can be used to connect applications to Internet Identity by Dfinity. (github.com)

main code here:
Web-Identity-Providers/source/src at main ¡ functionland/Web-Identity-Providers (github.com)

And used it here:
photos/auth.ts at main ¡ functionland/photos (github.com)
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