Looks like you get about 2k-4k viewers per video minimum, so maybe even just creating a 10-15 second bit that you can attach to the beginning or end of your videos talking about BuzzerBeater could get a decent amount of people interested or at least curious as to what it is. I think maybe one video addressing BB on it's own would also be beneficial, but I wouldn't drive too far away from your content/madden style. I would just mention it for 10-15 seconds at the beginning/end of your videos and link BB in your description or something! I'm sure you could easily get 50+ people interested and sign up for BB just after the first few videos, with 4k subscribers.
Murray/Harris/MPJ/Grant/Jokic - 2020 NBA Champs