If your status with Apple.com has changed to SHIPPED, it will work. If it's not yet shipped, there's no way it will work because there is no shipment to find.
Wow I just tried track by reference and it worked! I was a little worried that my status was still at preparing for shipment. Looks like my 6s Plus left China on the 19th. I am getting excited.
I wouldn't worry about it. I ordered two unlocked iPhone 6s Pluses and both are on my support profile. Only one, however, has a tracking number when I do the track by reference on either UPS or FedEx. Each phone was ordered with a different phone number so I tried both with each shipper and only got a tracking number for my rose gold 128gb.