Adding Custom Fields to Identity User in ASP.NET Identity

public class ApplicationUser : IdentityUser
{
    public string CustomTag { get; set; }
}

Use the ApplicationUser type as a generic argument for the context:

public class ApplicationDbContext : IdentityDbContext<ApplicationUser>
{
    public ApplicationDbContext(DbContextOptions<ApplicationDbContext> options)
        : base(options)
    {
    }

    protected override void OnModelCreating(ModelBuilder builder)
    {
        base.OnModelCreating(builder);
    }
}

Update Pages/Shared/_LoginPartial.cshtml and replace IdentityUser with ApplicationUser:

@using Microsoft.AspNetCore.Identity
@using WebApp1.Areas.Identity.Data
@inject SignInManager<ApplicationUser> SignInManager
@inject UserManager<ApplicationUser> UserManager

Update Areas/Identity/IdentityHostingStartup.cs or Startup.ConfigureServices and replace IdentityUser with ApplicationUser.

services.AddDefaultIdentity<ApplicationUser>(options => options.SignIn.RequireConfirmedAccount = true)
        .AddEntityFrameworkStores<ApplicationDbContext>();

References
https://docs.microsoft.com/en-us/aspnet/core/security/authentication/customize-identity-model?view=aspnetcore-6.0#custom-user-data

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