I tried

public class Startup : FunctionsStartup
{
    public override void Configure(IFunctionsHostBuilder builder)
    {
        var azureAdConfig = new ConfigurationBuilder()
            .AddInMemoryCollection(new Dictionary<string, string>
            {
                {"AzureAd:Instance", "https://login.microsoftonline.com/"},
                {"AzureAd:ClientId", "xxx"},
                {"AzureAd:TenantId", "xxx"}
            })
            .Build();

        builder.Services.AddAuthentication(sharedOptions =>
            {
                sharedOptions.DefaultScheme = JwtBearerDefaults.AuthenticationScheme;
                sharedOptions.DefaultChallengeScheme = JwtBearerDefaults.AuthenticationScheme;
            })
            .AddMicrosoftIdentityWebApi(azureAdConfig)
            .EnableTokenAcquisitionToCallDownstreamApi()
            .AddInMemoryTokenCaches();

and in the function

[Authorize]
[FunctionName("getStuff")]
public async Task<IActionResult> Run(
    [HttpTrigger(AuthorizationLevel.Anonymous, "get", "post", Route = null)] HttpRequest req,
    ILogger log)

I’m passing the token from the front but I get a 500
and in the console (backend) I see

[2024-06-06T10:37:58.854Z] An unhandled host error has occurred.

[2024-06-06T10:37:58.855Z] Microsoft.AspNetCore.Authentication.Core: No authentication handler is registered for the scheme ‘WebJobsAuthLevel’. The registered schemes are: Bearer. Did you forget to call AddAuthentication().AddSomeAuthHandler?.

[2024-06-06T10:37:59.279Z] Host lock lease acquired by instance ID ‘0000000000000000000000006A9C9842’.