How to set the display name for a Firebase user from the full name of an Apple ID

I have a Firebase iOS app and I've enabled sign up using an Apple account.

How do I set the display name for the Firebase user from the full name of the Apple ID credential?

This is how I get the full name from the Apple ID credential after the user as been authorized.

ASAuthorizationAppleIDCredential *appleIDCredential = authorization.credential;
NSString *displayName = appleIDCredential.fullName;