The Settings tab is where you configure Booya! Web Extension.
This is the list of domain names that you will use to create aliases. It is a comma-separated list of domains, i.e. my-alias-domain.com, other-domain.net.
These domains need to be registered with Gandi.net.
At Gandi.net this is called the Production API key. It is used to authenticate the API calls made by Booya! If you do not have one yet you can generate (a new) one in your account Security settings: https://account.gandi.net/en/users/<GANDI HANDLE>/security.
Make sure to store this somewhere secure since it is only displayed when it is first generated.
This is the email to which emails sent to your aliases will be redirected to. Normally this would be the email that you want to keep private.