On my Pro Box I opened the Discovery+ app and I was presented with the registration page with QR code and pin. Discovery+ is part of my package so why was registration needed. I aborted the process.
Discovery+ have their own customer base so they need to know who is paying the bill, they need to know that you are entitled to discovery+ and what content you are contracted for.
You will need to go into your online myBT account and activate discovery+ and then sign up for or into a discovery+ account.
Here's the link https://www.bt.com/sport/discovery-plus