Download the package from Package Manager inside Unity
Import it in your project
Add Google's Game Package Registry for Unity to your project: https://developer.android.com/games/develop/build-in-unity#download-registry .
After adding Game Package Registry, you should import Google Play Core package to your project from Unity. Open menu Windows -> Package Manager ,from the drop down menu on top left part of the Package Manager windows select 'My Registries' and Install 'Google Play Core'.
And that's all, you are good to go!