The farmer's hat add-on is an item that was obtainable from Treasure Hunter from 12 June 2015 0:00 UTC to 15 June 2015 23:59 UTC, and was re-added permanently on 20 August 2015. It can also be obtained as a reward from Cabbage Facepunch Bonanza. It can be used on a farmer's hat to create a modified farmer's hat. Once the add-on has been added to the farmer's hat, it cannot be undone.
As of 29 Jan 2018, the farmer's hat add-on can be purchased from the Cabbage Facepunch Bonanza reward shop.