The argument type 'Future Function(String?)' can't be assigned to the parameter type 'String? Function(String?)?'

Question

I am currently using flutter_login 3.1.0 in my application. During the process of user signing up, it is required that he assigns himself a username, the application is supposed to check whether the username that is typed in is taken or not from Firebase Firestore and display it to the user.

    @override
    Widget build(BuildContext context) {
        return FlutterLogin(
            // irrelevant code removed
            additionalSignupFields: [
              UserFormField(
                keyName: 'username',
                displayName: 'Username',
                icon: const Icon(FontAwesomeIcons.at),
                fieldValidator: (value) async {   // error 'Cant assign async function to function'
                  return await DatabaseService.validateUsername(value);
                },
              ),
            ]
        )
    }

The function assigned to fieldValidator has to be async because getting data from firestore always returns a Future.

People have told me to use FutureBuilder or StreamBuilder, I have tried and failed to achieve the functionality. It would great if code is provided rather than just an explanation