how can I pass variables to the register form?

The controller that returns the view showing the register form would also send variables to the view.

https://laravel.com/docs/5.8/views#passing-data-to-views

hi, i know but i will pass a list of country to registerform. I cant find the controller to pass the variable per compact to view.

All controllers related to auth are in /app/Http/Controllers/Auth. You're looking for the RegisterController. You'll see that controller is using a Trait called RegistersUsers. In that trait, you'll find a method called showRegistrationForm().

public function showRegistrationForm()
{
    return view('auth.register');
}

So what you'd do is copy that method into the RegisterController, and alter it how you need. It will then use yours instead of the default one in the trait.