Newtonsoft JSON for Unity3D

1) Open the manifest.json file located in:
                 <Your Project Folder>\Packages\manifest.json

2) Add this line:
            "com.unity.nuget.newtonsoft-json": "2.0.0"

The package will then be added by Unity:

Done!

To see  how to use Newtonsoft JSON refer to this post:
https://mydevelopertidbits.blogspot.com/2019/06/how-to-serialize-and-deserialize-json.html