This achievement is for honking the horn of any tractor.
You can set up your own game but it must be open/ public to unlock the achievement. Simple start up at match of any type with at-least one other player or bot. Choose the map Countryside (uncheck all the other maps beside Countryside) Once the match loads find a tractor and jump in the seat, now hit X to honk the horn.
Special thanks to Survivalist for pointing me in the right direction with this on.