How do I add custom claims to the accessToken?

[secret-agent-B]

I authenticate google users using the code below. But I am wondering, how do I add custom claims to the user so that when they I use the accessToken with custom claims to authorize requests going to my .NET Core API?

export interface SocialLoginResult {
  accessToken?: string;
  refreshToken?: string;
  displayName?: string | null;
  email?: string | null;
  photoURL?: string | null;
}

export const googleLoginAsync = async () : Promise<SocialLoginResult> => {
    const result = await FirebaseAuthentication.signInWithGoogle();
    const credential = GoogleAuthProvider.credential(
      result.credential?.idToken
    );

    const auth = getAuth();
    auth.currentUser?.getIdTokenResult()
      .then(idToken => {
        idToken.claims
      })
    const userCreds = await signInWithCredential(auth, credential);

    return {
      accessToken: (userCreds.user as { accessToken?: string }).accessToken,
      refreshToken: userCreds.user.refreshToken,
      displayName: userCreds.user.displayName,
      email: userCreds.user.email,
      photoURL: userCreds.user.photoURL,
    };
  }
);

[robingenz]

As far as I know, this is not possible with the Firebase Client SDKs. The Firebase Admin SDK offers a possibility: https://firebase.google.com/docs/auth/admin/custom-claims?hl=en