Laravel register redirect with data

I'm using the default Laravel Auth model and I'm looking to redirect the user after registration to a custom URL, and pass a variable from the form to that url. This data is not stored in the database with the user and will only be ever used on this "next step" url.

I've tried the following on the RegisterController;

protected function redirectTo(Request $request)
{
    return view('planBilling', ['plan' => $request->input('plan')]);
}

But the plan data is unknown.

via Chebli Mohamed