If you’ve got your own gateway, then peek in its logs (or even easier: see the gateway’s Traffic page in TTN Console) to see if it’s trying to do an OTAA Join when powering up. If not, then you’re out of luck as you cannot use a pre-programmed ABP DevAddr.
(As an aside: staging.thethingsnetwork.org should no longer be used, if it’s still operational to start with.)