I'm not sure if this is the case or not
It surely is, your case is nothing special: you are simply asking how to reactivate a scenery, and these are the only information you need.
The only thing you need is your Serial Number, which is indicated in your purchase receipt. The fact you bought it at a discount or even got if for free because of a special promotion at that time, doesn't mean you don't have a Serial Number and a proper purchase receipt for it.
I don't think I had that activation keys for SZH at the beginning
Yes, you had, in the purchase receipt you got via email. The above explanation are useful to retrieve it if you lost that email.
I bought ORD,JFK and the system automatic "activate" the SZH
If you purchased using the integrated purchase method, the sceneries are automatically activated without having to manually insert any Serial Number. This doesn't mean you don't have a Serial Number, it's just it was automatically inserted for you during purchase.
But when you need to *reinstall*, you have to insert you Serial Number, using the "Register Serial" option of the Addon Manager, for each scenery you have to reinstall. Please do as instructed, and you'll find you have purchase receipt for all your sceneries (regardless if you payed for them or not), and each one will contain a Serial Number.