Passing IHttpClientFactory to .NET Standard class library

First, your library class' constructor should take an HttpClient param, so you can inject an HttpClient into it. Then, the easiest method (mentioned in the link article as well for what it's worth) is to simply add a specific HttpClient for that library class:

services.AddHttpClient<MyLibraryClass>(...);

Then, of course, register your library class for injection, if you haven't already:

services.AddScoped<MyLibraryClass>();

Then, when your library class is instantiated to be injected into something, it too will be injected with the HttpClient you specified for it.

Alternatively, you can manually specify an HttpClient instance to inject via:

services.AddScoped(p => {
    var httpClientFactory = p.GetRequiredService<IHttpClientFactory>();
    return new MyLibraryClass(httpClientFactory.Create("Foo"));
});

Nowadays there is a NuGet package Microsoft.Extensions.Http offering the IHttpClientFactory to .NET Standard 2.0