How to install FSharp.xUnit? - f#

Question

I've installed Visual Studio 2017 and included the test package and the F# package. I'm trying to run a simple test using xUnit but it says xUnit is not found.
module HelloWorldTest
open FsUnit.xUnit
open Xunit
open System.Diagnostics
[<Fact>]
let ``Say Hello`` () =
Assert.True(true)
Error
The namespace or module 'FsUnit' is not defined.
I tried installing the package with dotnet but that produces an error.
> dotnet add package FSharp.xUnit
Writing C:\Temp\tmp8DAB.tmp
info : Adding PackageReference for package 'FSharp.xUnit' into project 'C:\Users\mjohn\workspace\fsharp\hello-world\HelloWorld.fsproj'.
log : Restoring packages for C:\Users\mjohn\workspace\fsharp\hello-world\HelloWorld.fsproj
...
error: Unable to find package FSharp.Core. No packages exist with this id in source(s): Microsoft Visual Studio Offline Packages
error: Unable to find package FsUnit.xUnit. No packages exist with this id in source(s): Microsoft Visual Studio Offline Packages
error: Unable to find package xunit. No packages exist with this id in source(s): Microsoft Visual Studio Offline Packages
error: Unable to find package xunit.runner.visualstudio. No packages exist with this id in source(s): Microsoft Visual Studio Offline Packages
error: Unable to find package FSharp.xUnit. No packages exist with this id in source(s): Microsoft Visual Studio Offline Packages
error: Unable to find package Microsoft.NET.Test.Sdk with version (>= 16.0.0)
error: - Found 1 version(s) in Microsoft Visual Studio Offline Packages [ Nearest version: 15.9.0]
error: Package 'FSharp.xUnit' is incompatible with 'all' frameworks in project 'C:\Users\mjohn\workspace\fsharp\hello-world\HelloWorld.fsproj'.
So how can I install xUnit for F#? This guide from MS unfortunately is lacking any of this information.
https://learn.microsoft.com/en-us/dotnet/core/testing/unit-testing-fsharp-with-dotnet-test
EDIT:
After adding the NuGet package source
> dotnet add package FsUnit.xUnit
Writing C:\Temp\tmp5BA6.tmp
info : Adding PackageReference for package 'FsUnit.xUnit' into project 'C:\Users\mjohn\workspace\fsharp\hello-world\HelloWorld.fsproj'.
log : Restoring packages for C:\Users\mjohn\workspace\fsharp\hello-world\HelloWorld.fsproj
...
info : CACHE https://api.nuget.org/v3-flatcontainer/fsunit.xunit/index.json
info : Package 'FsUnit.xUnit' is compatible with all the specified frameworks in project 'C:\Users\mjohn\workspace\fsharp\hello-world\HelloWorld.fsproj'.
info : PackageReference for package 'FsUnit.xUnit' version '3.4.0' updated in file 'C:\Users\mjohn\workspace\fsharp\hello-world\HelloWorld.fsproj'.