Unable to set User FK in another model (User extends IdentityUser)

  // Study.cs
  public class Study
  {
    public int Id { get; set; }
    //...
    public required string UserId { get; set; }
    public IdentityUser? User { get; set; }
  }

  // User.cs
    public class User : IdentityUser
  {
    public List<Study> Studies { get; set; } = new List<Study>();
  }

  public class ApplicationDbContext : IdentityDbContext<IdentityUser>
  {
    public DbSet<User> Users { get; set; }
    public DbSet<Study> Studies { get; set; }

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

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

      builder.Entity<Study>()
      .HasOne(s => s.User)
      .WithMany(u => u.Study)
      .HasForeignKey(s => s.UserId)
    }
  }