Program.cs

using Microsoft.AspNetCore.SystemWebAdapters;

var builder = WebApplication.CreateBuilder(args);
builder.Services.AddReverseProxy().LoadFromConfig(builder.Configuration.GetSection("ReverseProxy"));

builder.Services.AddControllers();

builder.Services.AddSystemWebAdapters()
    .AddRemoteAppClient(options =>
    {
        options.RemoteAppUrl = new(builder.Configuration["ReverseProxy:Clusters:fallbackCluster:Destinations:fallbackApp:Address"]);
        options.ApiKey = builder.Configuration["RemoteAppApiKey"];
    })

    // This registers the remote app authentication handler. The boolean argument indicates whether remote app auth
    // should be the default scheme. If it is set to false, HTTP requests to authenticate will only be made for
    // endpoints that actually need that behavior, but it is then necessary to annotate endpoints requiring remote app
    // auth with [Authorize(AuthenticationSchemes = RemoteAppAuthenticationDefaults.AuthenticationScheme)] or something similar.
    .AddAuthenticationClient(isDefaultScheme: true);

var app = builder.Build();

app.UseHttpsRedirection();

app.UseAuthentication();
app.UseAuthorization();

app.MapControllers();

app.MapReverseProxy();

app.Run();