Problem with play-games-plugin-for-unity plugin. How to decide?

Question

Unity

There is an application, it has its own Backend and web-api for interacting with it. In order to work with it, I need to get the server auth code (PlayGamesPlatform.Instance.GetServerAuthCode()). However, for this it is necessary to perform authorization, but it does not pass (goes endlessly).
As I understand it, the problem is that the apk file that deploys the application is not certified and authorization fails.
The catch is that the project is freelancing (and I am the executor of the order), and they do not want to provide me with the upload key on their Google Console, because, as I was told, it matches the application key in the Google Console.
As I understand it, having an upload key that the downloader will create, I will be able to upload an APK file to them for internal testing, signed with the upload key, and the Google Console will change the signature itself so that it is considered as an application from the customer?
Or are there other solutions to the problem and am I digging in the wrong direction?

Answer 1

No, there are no options.
In order for your application / game to be authorized through Google Play Games, you need
1. For the application to be signed with the same key that it is signed in the store.
2. The application had play services in the console.
3. In your case, in addition to the resources of the services, you must also add the Web client ID, it is also obtained in the Google console.
About signatures. Signatures that were made during the first upload to the store can no longer be changed.
The key is created alone, the application is signed by it. The application is sent to the store.
All they can do now is create a second application in the console, configure everything, send you a key, you test it, and then they just rebuild the application with their old key and upload this build to the old console application profile. But it's like that.)