...
using System.Security.Claims;

namespace RazorAuthSample.Areas.Identity.Pages.Account
{
    [AllowAnonymous]
    public class RegisterModel : PageModel
    {
        ...
        
        public async Task<IActionResult> OnPostAsync(string returnUrl = null)
        {
            ...
            if (ModelState.IsValid)
            {
                var user = new IdentityUser { UserName = Input.Email, Email = Input.Email };
                var result = await _userManager.CreateAsync(user, Input.Password);
                if (result.Succeeded)
                {
                    var claims = new List<Claim> 
                    {
                        new Claim("FirstName", Input.FirstName),
                        new Claim("IsAdmin", Input.IsAdmin),
                    };
                    await _userManager.AddClaimsAsync(user, claims);

                    // To add a single claim
                    // var claim = new Claim("FirstName", Input.FirstName);
                    // await _userManager.AddClaimAsync(user, claim);
                    ...
                }
            }
            
            // If we got this far, something failed, redisplay form
            return Page();
        }
     }
 }