I created a class that extended BackOfficeSignInManager and overrode it's PasswordSignInAsync(string, string, bool, bool) function. You will also need to include System.DirectoryServices.AccountManagement from NuGet.

public class ActiveDirectorySignInManager : BackOfficeSignInManager
{
    // there will be a couple constructors that must be present

    public override async Task<SignInResult> PasswordSignInAsync(string username, string password, bool isPersistent,
        bool lockoutOnFailure)
    {
        // umbraco admin account does not use AD.
        if (username == "admin")
            return await base.PasswordSignInAsync(username, password, isPersistent, lockoutOnFailure);

        // validate user in AD
        using var pc = new PrincipalContext(ContextType.Domain, "domain");
        using var adUser = UserPrincipal.FindByIdentity(pc, username);
        if (adUser == null || !pc.ValidateCredentials(username, password))
            return SignInResult.Failed;

        // creates a user account in Umbraco
        var user = await UserManager.FindByNameAsync(username);
        if (user == null)
        {
            var createResult = await UserManager.CreateAsync(BackOfficeIdentityUser.CreateNew(_globalSettings.Value,
                username, "[email protected]",
                "en-US", adUser.Name));

            if (!createResult.Succeeded)
                return SignInResult.Failed;

            user = await UserManager.FindByNameAsync(username);

            if (user == null)
                return SignInResult.Failed;
        }

        await base.SignInAsync(user, false);
        return SignInResult.Success;
    }
}

Then in the Startup.cs ConfigureServices method, I added a Replace.

public void ConfigureServices(IServiceCollection services)
{
    services.AddUmbraco(_env, _config)
        .AddBackOffice()
        .AddWebsite()
        .AddComposers()
        .Build();

    services.Replace(new ServiceDescriptor(typeof(IBackOfficeSignInManager),
        typeof(ActiveDirectorySignInManager), ServiceLifetime.Transient));
}

It's not perfect, there's some functionality I still need to add, but it's a place to start.

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