Download GoogleService.Json File from google firebase console & add it in Platforms/Android folder & Set that file build action as GoogleServicesJson.
If you can not see this GoogleServicesJson Option. Then Install following plugin in your android project depedency.
Download GoogleService-Info.plist File from google firebase console & add it in Platforms/iOS folder & You need to set build action as BundleResource manually by editing project file.
<ItemGroup>
<BundleResource Include="Platforms\iOS\GoogleService-Info.plist" Link="GoogleService-Info.plist" />
</ItemGroup>