First: Create result pages for each quiz outcome on your website, include the word "result" in each outcome page URL
Set up a URL on your site for each of your quiz outcomes, then set up quiz outcome redirect so traffic goes to your URLs once someone completes the quiz.
Here's my URLs, you'll see the word "result" is in each URL.
And here's how the setup looks inside of Interact.
Note: When using Lead Generation for your quiz the results pages are seen after someone opts-in, we recommend leaving the opt-in optional, which means results can be seen without opting in. You can force the opt-in which means someone would only reach the results if they have opted in, but we do not recommend it.
Second: Create Custom conversion where URL contains “result”
Here's the doc on Setting up Meta Custom Conversion. You are going to set up a URL rule where the URL contains the word "result"
Now you will have a Custom Conversion Event that fires when someone reaches any outcome page of your quiz and if you have Lead Generation enabled on your quiz you will be able to track cost per lead using this method.