The season completion bonuses are awarded for completing the season specific events. They are the pr stunts, rival races, seasonal championships and all of the other specific requests listed under the seasonal activity list. The rewards are given out at 50% and 80% season completion for every season. The menu is in the Horizon life menu and the challenges are located under the season festival playlist.
Image of the menu button to click
Image of the reward you get for completing the tasks in the menu