You will need to use a map installer to install a GMAP format map that BaseCamp can read. Garmin has a separate map installer program that you will need to do this. I am not sure if BBBike offers maps in this format, but with a little googling, it shouldn't be too hard to find them already built. I honestly have found much better success building routes in Komoot or Strava and then sending them to my Garmins over the years rather than using BaseCamp, primarily because those options offer better routing and there is the advantage of using heat maps (in Strava) to stick to roads that cyclists use.