In League Settings → Schedule, edit the race and open Race Sponsor. Pick a sponsor you've used before, or choose Add a new sponsor and fill in its artwork (a logo, or a WebM/MP4 bumper), name, website, broadcast placement, time on screen and broadcast notes. Then type the ad read for this race, word for word.
A race sponsor belongs to that race only. It isn't added to your season-long sponsor list or your stream descriptions. Its artwork shows on the overlays during that race once your broadcaster approves it, and they can approve it in the same window. Its ad read appears at the top of the commentators' AD Read window in the GridCast Broadcaster. Choose Ad read only if it shouldn't appear on the overlays.
