It's just like using the Cloud Code Explorer (Design | Cloud Code | API Explorer) - and performing an Authenticate Service, Authenticate Operation call - with the default parameters. That is - it will use the email address you logged in as both your email and password.
If, however, you have created an account at that email with a different password, the Quick Authenticate will fail... (because it is using the wrong password). And you will get error 40307 | TOKEN_DOES_NOT_MATCH_USER | The user credentials don't match (i.e. incorrect password).
This gives you a couple of options:
1 - Change the password of that user account. You can do that via the API Explorer or User Monitoring. You can also simply delete the user account - and then it will be recreated the next time.
2 - The "session" is shared between the API Explorer and Cloud Code Script Editor. So you *could* login via the API Explorer (passing in the correct password), and *then* go to the Cloud Code script and hit run. (You won't need to hit Quick Authenticate, cause you have already authenticated.).