I posted this in another thread, but I'm going to re-share it here for people to see: If you have an existing fitbit account any free trials start at the moment of purchase. However, if you don't have a fitbit account (or didn't place your order using your account) and set one up while activating a new device, your trial will start at the moment of activation. For example, if I gift someone a fitbit when they activate it they'll get the option to start a premium trial if they weren't already a premium user.
I recently had to go through this when I bought my mom a Sense. Because neither of us had Premium, and I was purchasing it through my fitbit account, we both ended up with Premium trials. I think they set up all the devices to re-offer the trial activation at set-up, so people buying at different retail locations (such as Best Buy) wouldn't miss out on the trials--but it only works if you don't already have premium, and ordering directly through fitbit prompts the premium trial to start automatically.
It might be better to call. I haven't tried calling, but I've seen people claim it's easier to get help. I've noticed that on the chat and email the support techs aren't fully fluent in English and instead are reading through prompts, so anything that is more nuanced or not covered by the prompts they were given is hard to get help for.