app/Http/Controllers/Auth/AuthController.php
protected function validateCoupon($validator, Request $request)
    {
        try {
            $coupon = StripeCoupon::retrieve(
                $request->coupon, ['api_key' => config('services.stripe.secret')]
            );

            if ($coupon && $coupon->valid) {
                return;
            }
        } catch (Exception $e) {
            //
        }

        $validator->errors()->add('coupon', 'The provided coupon code is invalid.');
    }
protected function validateCoupon($validator, Request $request)
    {
        try {
            $coupon = StripeCoupon::retrieve(
                $request->coupon, ['api_key' => config('services.stripe.secret')]
            );

            if ($coupon && $coupon->valid) {
                return;
            }
        } catch (Exception $e) {
            //
        }

        $validator->errors()->add('coupon', 'The provided coupon code is invalid.');
    }
Validate that the provided coupon actually exists on Stripe.

@param  \Illuminate\Validation\Validator  $validator
@param  \Illuminate\Http\Request  $request
@return void