How to change authentication mode (from Windows to Azure AD)

I have an existing WebForms application that currently uses Windows Authentication that I want to authenticate through Azure AD. 

Here is a working example for AAD Authentication with Web Forms.

Web.config

<configuration>
<!-- Azure AD Settings -->
  <appSettings>
    <add key="ida:ClientId" value="{ClientId}" />
    <add key="ida:AADInstance" value="https://login.microsoftonline.com/" />
    <add key="ida:Domain" value="{Tenant}" />
    <add key="ida:TenantId" value="{TenantId}" />
    <add key="ida:PostLogoutRedirectUri" value="https://localhost:44306/" />
  </appSettings>
  <location path="Account">
    <system.web>
      <authorization>
        <allow users="*" />
      </authorization>
    </system.web>
  </location>
  <system.web>
    <!-- Request Login -->
    <authorization>
      <deny users="?" />
    </authorization>

    <authentication mode="None" />
    <compilation debug="true" targetFramework="4.5" />
    <httpRuntime targetFramework="4.5" />
    <pages>
      <namespaces>
        <add namespace="System.Web.Optimization" />
      </namespaces>
      <controls>
        <add assembly="Microsoft.AspNet.Web.Optimization.WebForms" namespace="Microsoft.AspNet.Web.Optimization.WebForms" tagPrefix="webopt" />
      </controls>
    </pages>
    <httpModules>
      <add name="ApplicationInsightsWebTracking" type="Microsoft.ApplicationInsights.Web.ApplicationInsightsHttpModule, Microsoft.AI.Web" />
    </httpModules>
  </system.web>
  <system.webServer>
    <!-- Remove Forms Authentication Module. -->
    <modules>
      <remove name="FormsAuthentication" />
      <remove name="ApplicationInsightsWebTracking" />
      <add name="ApplicationInsightsWebTracking" type="Microsoft.ApplicationInsights.Web.ApplicationInsightsHttpModule, Microsoft.AI.Web" preCondition="managedHandler" />
    </modules>

    <validation validateIntegratedModeConfiguration="false" />
  </system.webServer>
  <runtime>
    <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
      <dependentAssembly>
        <assemblyIdentity name="Newtonsoft.Json" culture="neutral" publicKeyToken="30ad4fe6b2a6aeed" />
        <bindingRedirect oldVersion="0.0.0.0-6.0.0.0" newVersion="6.0.0.0" />
      </dependentAssembly>
      <dependentAssembly>
        <assemblyIdentity name="WebGrease" culture="neutral" publicKeyToken="31bf3856ad364e35" />
        <bindingRedirect oldVersion="0.0.0.0-1.5.2.14234" newVersion="1.5.2.14234" />
      </dependentAssembly>
    </assemblyBinding>
  </runtime>
  <system.codedom>
    <compilers>
      <compiler language="c#;cs;csharp" extension=".cs" type="Microsoft.CodeDom.Providers.DotNetCompilerPlatform.CSharpCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=1.0.8.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" warningLevel="4" compilerOptions="/langversion:6 /nowarn:1659;1699;1701" />
      <compiler language="vb;vbs;visualbasic;vbscript" extension=".vb" type="Microsoft.CodeDom.Providers.DotNetCompilerPlatform.VBCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=1.0.8.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" warningLevel="4" compilerOptions="/langversion:14 /nowarn:41008 /define:_MYTYPE=\&quot;Web\&quot; /optionInfer+" />
    </compilers>
  </system.codedom>
</configuration>

Startup.Auth.cs

public partial class Startup
{
    private static string clientId = ConfigurationManager.AppSettings["ida:ClientId"];
    private static string aadInstance = ConfigurationManager.AppSettings["ida:AADInstance"];
    private static string tenantId = ConfigurationManager.AppSettings["ida:TenantId"];
    private static string postLogoutRedirectUri = ConfigurationManager.AppSettings["ida:PostLogoutRedirectUri"];
    string authority = aadInstance + tenantId;
    public void ConfigureAuth(IAppBuilder app)
    {
        app.SetDefaultSignInAsAuthenticationType(CookieAuthenticationDefaults.AuthenticationType);
        app.UseCookieAuthentication(new CookieAuthenticationOptions());
        app.UseOpenIdConnectAuthentication(
            new OpenIdConnectAuthenticationOptions
            {
                ClientId = clientId,
                Authority = authority,
                PostLogoutRedirectUri = postLogoutRedirectUri,
                Notifications = new OpenIdConnectAuthenticationNotifications()
                {
                    AuthenticationFailed = (context) =>
                    {
                        return System.Threading.Tasks.Task.FromResult(0);
                    }
                }
            }
            );
        // This makes any middleware defined above this line run before the Authorization rule is applied in web.config
        app.UseStageMarker(PipelineStage.Authenticate);
    }
}