Xamarin In-App Billing Plugin subscription products for 1 month free promotion questions

Question

I'd like to sell Subscriptions products on Apple.

And when Ios made a subscription product, I set up a monthly free promotion.

By the way, can't I check the payment for free for a month when I pay for the Subscription products with my Sandbox account?

I'm curious because if I pay for the subscription product with my sandbox account, I can't see the free payment for a month.

Or should I write something on the code shown below?

bool OnShouldAddStorePayment(SKPaymentQueue queue, SKPayment payment, SKProduct product)
{
     return true;
}