Try IdentityDbContext<ExtendedUser>
as your base class for UsersContext
as it is in the post you have mentioned. Without it you don't have any line of code to tell ef which property is the key. If you don't want IdentityDbContext you should in method void OnModelCreating(DbModelBuilder modelBuilder)
put property mapping, something like:
protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
base.OnModelCreating(modelBuilder);
modelBuilder.Entity<ExtendedUser>().HasKey(t => t.Id);
...
}