There is a different code for each metropolitan location where it is valid. I currently only know of the following:
San Francisco: HORNITOSSF
San Diego: HORNITOSSD
Dallas: HORNITOSDAL
Your ride will be free if it’s <$10. Promotion expires Sunday.
If your location is not listed, try just opening the app. You might see a pop up window advertising the promotion (that’s how I found this).