I have a Google API key that is restricted to only an iOS app with a specific bundle identifier.
For that key to work, the GET request to the API needs to include this header x-ios-bundle-identifier as mentioned in this StackOverflow comment
Here is the place in the code where you can add this

Something like this:
Map<String, String> headers = {};
if (defaultTargetPlatform == TargetPlatform.iOS) {
headers['x-ios-bundle-identifier'] = "com.example.app";
}
Response response =
await _dio.get(url, options: Options(headers: headers));