Surface and add IHttpClientFactory to example code (#17581)

Author: guardrex

aspnetcore/security/blazor/server.md

@@ -5,7 +5,7 @@ description: Learn how to mitigate security threats to Blazor Server apps.
 monikerRange: '>= aspnetcore-3.1'
 ms.author: riande
 ms.custom: mvc
-ms.date: 03/31/2020
+ms.date: 04/02/2020
 no-loc: [Blazor, SignalR]
 uid: security/blazor/server
 ---
@@ -100,6 +100,10 @@ The command creates a folder named with the value provided for the `{APP NAME}`
 Authenticate the Blazor Server app as you would with a regular Razor Pages or MVC app. Provision and save the tokens to the authentication cookie. For example:
 
 ```csharp
+using Microsoft.AspNetCore.Authentication.OpenIdConnect;
+
+...
+
 services.Configure<OpenIdConnectOptions>(AzureADDefaults.OpenIdScheme, options =>
 {
     options.ResponseType = "code";
@@ -137,9 +141,13 @@ public class TokenProvider
 }
 ```
 
-In `Startup.ConfigureServices`, register the token provider service:
+In `Startup.ConfigureServices`, add services for:
+
+* `IHttpClientFactory`
+* `TokenProvider`
 
 ```csharp
+services.AddHttpClient();
 services.AddScoped<TokenProvider>();
 ```