How to get mail from toMail method which is in PasswordResetNotification in Laravel 6.2

I'm using Laravel 6.2. How can I get the reset mail from password reset form inside PasswordResetNotification's toMail method? There I have a url to redirect , like below. . .

public function toMail($notifiable) {
    $url_to_reset = "http://localhost:3000/admin/reset-password/client-mail=" . $user_mail . "&token=" . $this->token;
    return (new MailMessage)->from('mymail@gmail.com');
}

I want to get $user_mail. How can I get it? Thank you.