Accessing On Premise Api Through Azure Application Proxy with Azure Directory Enabled Doesn't Work as Documentation Describes

Question

I'm attempting to expose my on-premise APIs with Azure Application Proxy.

I've successfully configured Azure Application Proxy to work with the Pre-Authentication set to Passthrough. When I change the Pre-Authentication to Azure Active Directory I can access the endpoint successfully via a browser. However, when I try calling the on-premise endpoint from code I receive the HTML for the Microsoft Sign-In page. A successful request will return a JSON Response.

I'm following a Microsoft article: Secure access to on-premise APIs with Azure AD Application Proxy

Via an Azure Docs defect, I learned that I have to use "http://localhost" as a value for the RedirectUri and configure my client app as a "Mobile and desktop applications" platform.

[Fact]  
public async Task Successfully_authenticate_but_cant_access_the_proxy()  
{  
    // Acquire Access Token from AAD for Proxy Application  
    var clientApp = PublicClientApplicationBuilder  
        .Create("b510069b-xxxx-xxxx-xxxx-9363xxxxxxxx") //Client Id for Client Application  
        .WithRedirectUri("http://localhost") // This must be configured as a "Mobile and desktop applications" platform in the client application  
        .WithTenantId("xxxxxx-d4cf-4xxx-xxxx-8dc72cbc00bd") //Not sure if this is needed.  
        .WithAuthority("https://login.microsoftonline.com/xxxxxx-d4cf-4xxx-xxxx-8dc72cbc00bd/oauth2/v2.0/authorize")  
        .Build();  
  
    AuthenticationResult authResult;  
    var accounts = await clientApp.GetAccountsAsync();  
    var account = accounts.FirstOrDefault();  
  
    IEnumerable<string> scopes = new string[] {"https://endpoints-xxx.msappproxy.net/user_impersonation"};  
  
    try  
    {  
        authResult = await clientApp.AcquireTokenSilent(scopes, account).ExecuteAsync();  
    }  
    catch (MsalUiRequiredException ex)  
    {  
        authResult = await clientApp.AcquireTokenInteractive(scopes).ExecuteAsync();                  
    }  
  
    if (authResult != null)  
    {  
        //Use the Access Token to access the Proxy Application  
        var httpClient = new HttpClient();  
        httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", authResult.AccessToken);  
        var response = await httpClient.GetAsync("https://endpoints-xxx.msappproxy.net");  
  
        //Failing here. I'm receiving the HTML for the Sign-In page. I'm expecting a response with JSON.  
        var responseValue = await response.Content.ReadAsStringAsync();  
    }  
}