You you do have to have a TrainerRoad subscription. You can sign up here:
Once you get a TrainerRoad account you can go into your Settings and set up your FTP(functional threshold power), your weight and your age. If you have never done an FTP test they will estimate for you. Please go here for more information. You can also do the ramp test by yourself on TrainerRoad to determine your FTP.
You can’t adjust your FTP during a workout but you can always adjust the % to be higher or lower than 100% in the lower left corner of the screen. here