Having had a look at the NOW help page I don’t think Roku is supported for bonus streams yet.
Got a NOW Sports Membership, and one of the supported devices listed below? You can access extra channels on selected sports events using Bonus Streams, including additional EFL matches and F1 Race Control.
You’ll find all available Bonus Streams at the bottom of your NOW app homepage. Don't worry if you can't see any Bonus Streams right now – they'll appear automatically by the time the event starts.
You can’t access Bonus Streams with a Sports Day or Week Membership. NOW Boost features, including full HD and 50 frames per second, aren't available on Bonus Streams.
Updated Now TV app with bonus stream support was added to both my Roku devices recently (perhaps within the last 2-3 weeks). I have a Roku Streaming Stick+ and a Roku Express, both bought in 2018.