Azure AAD - Application Proxy to On-Premise Service - azure

Question

I've been trying to call an on-premise Api from another Api in Azure, using Application Proxy. I've followed several examples like the one listed below, with similar results. I can successfully acquire an access token but when I make an http call to on-prem Api via app proxy, I always get redirected to the sign-in page even though I've added an authorization header with bearer token. Anybody have thoughts on what I may be missing?
https://blogs.technet.microsoft.com/applicationproxyblog/2018/06/27/securing-api-access-beyond-intranet-with-azure-ad-application-proxy/
This is the code I'm running that was copied from the referenced example. The one thing I'm not 100% on is the todoListResourceId & todoListBaseAddress. I have them as the same value which is the "Homepage URL" of the configured Application Proxy app.
AuthenticationContext authContext;
private static string aadInstance = "https://login.microsoftonline.com/{0}";
private static string tenant = "xxx.onmicrosoft.com";
private static string clientId = "my app id";
Uri redirectUri = new Uri("http://replyurl");
private static string authority = String.Format(CultureInfo.InvariantCulture, aadInstance, tenant);
private static string todoListResourceId = "https://xxx.msappproxy.net";
private static string todoListBaseAddress = "https://xxx.msappproxy.net";
private async void GetTodoList()
{
AuthenticationResult result = null;
HttpClient httpClient = new HttpClient();
authContext = new AuthenticationContext(authority);
result = await authContext.AcquireTokenAsync(todoListResourceId, clientId, redirectUri, new PlatformParameters(PromptBehavior.Auto));
httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", result.AccessToken);
// Call the To Do list service.
HttpResponseMessage response = await httpClient.GetAsync(todoListBaseAddress + "/api/values/4");
//MessageBox.Show(response.RequestMessage.ToString());
string s = await response.Content.ReadAsStringAsync();
MessageBox.Show(s);
}