Assuming you have a SurfaceController or UmbracoController or something then I'd be injecting an IBackOfficeUserManager and using that.

Something like this should get you started:

using Microsoft.AspNetCore.Identity;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Extensions.Options;
using Umbraco.Cms.Core.Configuration.Models;
using Umbraco.Cms.Core.Security;

// change this namespace to your own
namespace Umbraco.Cms.Web.UI.Controllers;

public class SomethingController : UmbracoController
{
    private readonly IBackOfficeUserManager _backOfficeUserManager;
    private readonly GlobalSettings _globalSettings;

    public SomethingController(
        IBackOfficeUserManager backOfficeUserManager,
        IOptions<GlobalSettings> globalSettings)
    {
        _backOfficeUserManager = backOfficeUserManager;
        _globalSettings = globalSettings.Value;
    }

    public async Task<IActionResult> Index(string name, string email, string password)
    {
        var identityUser = BackOfficeIdentityUser.CreateNew(
            _globalSettings,
            email,
            email,
            _globalSettings.DefaultUILanguage);

        identityUser.Name = name;

        IdentityResult created = await _backOfficeUserManager.CreateAsync(identityUser, password);

        return created.Succeeded ? Ok() : BadRequest();
    }
}

That should at least get you started

is working on a reply...