After a player has attacked your Forest, they will be added to your "Revenge" list so you can attack them back if you wish! When you are awarded an Attack as a Chest Reward, the "Revenge" button will appear in the top, right-hand corner. You can either choose a player who previously attacked you or select the "Random" button to let the game randomly choose a player's Forest to Attack.

For Raids, a player's Forest is randomly assigned to you and there is not a way to pick.