可以在生成的Register.cshtml.cs文件中手动添加密码策略，修改OnPostAsync方法。具体实现可以通过使用PasswordOptions类来指定要求的密码复杂度，例如：

using Microsoft.AspNetCore.Identity;
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.RazorPages;

namespace MyApp.Areas.Identity.Pages.Account
{
    public class RegisterModel : PageModel
    {
        private readonly SignInManager _signInManager;
        private readonly UserManager _userManager;

        public RegisterModel(
            UserManager userManager,
            SignInManager signInManager)
        {
            _userManager = userManager;
            _signInManager = signInManager;
        }

        [BindProperty]
        public InputModel Input { get; set; }

        public string ReturnUrl { get; set; }

        public class InputModel
        {
            [EmailAddress]
            [Display(Name = "Email")]
            [Required]
            public string Email { get; set; }

            [DataType(DataType.Password)]
            [Display(Name = "Password")]
            [Required]
            public string Password { get; set; }

            [DataType(DataType.Password)]
            [Display(Name = "Confirm password")]
            [Compare("Password", ErrorMessage = "The password and confirmation password do not match.")]
            public string ConfirmPassword { get; set; }
        }

        public void OnGet(string returnUrl = null)
        {
            ReturnUrl = returnUrl;
        }

        public async Task OnPostAsync(string returnUrl = null)
        {
            returnUrl = returnUrl ?? Url.Content("~/");
            if (ModelState.IsValid)
            {
                var user = new IdentityUser { UserName = Input.Email, Email = Input.Email };
                var result = await _userManager.CreateAsync(user, Input.Password);

                if (result.Succeeded)
                {
                    // add password policy
                    var options = new PasswordOptions
                    {
                        RequiredLength = 8,
                        RequireLowercase = true,
                        RequireUppercase = true,
                        RequireDigit = true,
                        RequireNonAlphanumeric = true
                    };
                    var passwordValidator = new PasswordValidator();
                    passwordValidator.Configure(options);
                    _userManager.PasswordValidators.Add(passwordValidator);

                    await _signInManager.SignInAsync(user, isPersistent: false);
                    return LocalRedirect(returnUrl);
                }
                foreach (var error in result.Errors)
                {
                    ModelState.AddModelError(string.Empty, error.Description);
                }
            }

            // If we got this far, something failed, redisplay form
            return Page();
        }
    }
}

以上代码先创建一个PasswordOptions对象，并指定要求的密码长度和各种字符的使用情况，然后再使用PasswordValidator类和UserManager.PasswordValidators.Add()方法将密码策略配置到UserManager中。这样，在注册新用户时，会自动验证密码是否符合指定的复杂度要求。

注：以上示例代码中使用了ASP.NET Core Identity，需要在Startup.cs文件中添加相应的服务配置，如下所示：

services.AddDbContext(options =>
    options.UseSqlServer(
        Configuration.GetConnectionString("DefaultConnection")));
services.AddDefaultIdentity(options => options.SignIn.RequireConfirmedAccount = true)
    .AddEntityFrameworkStores();