How to add and use a settings.json file to my .NET Core Unit Test project

I have an existing solution with a few projects in it that are all .NET 4.6.2. One of them is a unit test project. None of the code is in production yet, and the entire solution is still in the early stages of development. We have decided to migrate it all to .NET Core before we get too far down the road.

I started the conversion to Core with the Unit Test project. The learning curve of Core is a little bit more than I expected, and I've been spending a lot of time on this website, reading other posts, to get things started.

The problem I'm having right now is figuring out how to read values from a config file. In the old 4.6.2 project, we had the familiar app.config with keys as such:

    <add key="APIUriRoot" value="http://localhost:26307/api" />

I am trying to figure out how to do the same in the new Core unit test project that I've created. Everything I find says something like:

"Just open up your settings.json file and add stuff! Simple!"

But... I don't HAVE a settgings.json file, and I can't figure out how to add one. I did add a normal old XML based app.config file, but when trying to read values from it:

_publicID = ConfigurationManager.AppSettings["APIPublicID"];

They come back null.

Here's a snapshot of each project:

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When I try to Add New to UnitTestsCore, I can't find any option or way to add a settings.json file.

How do I do this?