Identify promotional offer in webhook

First of all, thank you for your great work.

Is there a way to identify a webhook event is a promotional offer or not?

We have a promotional offer in apple store: first month 50% off discount. Webhook events look like this:

1. First event:

{

  …,

  "period_type": "NORMAL",

  “price_in_purchased_currency”: 10

}

2. Second event:

{

  …, 

  "period_type": "NORMAL",

  “price_in_purchased_currency”: 20

}

Which looks correct, however, we want to identify the first event as ‘promotional offer initial purchase` for our own record. It can be identified through ‘price_in_purchased_currency’ but it’ll get complicated since there are up to 200 currencies. 

We tried to add customer attribute to give more information when user buy promotional offer, but that goes to all events so it doesn’t really meet our needs.

Just wondering if there’s another way to differentiate the initial ‘promotional offer’ subscription from following ‘normal’ subscription. Thanks a lot!