Bottling bucket. Don't add too much though, it will be awful. I have a few cases from months back that I still can't drink. Don't just follow the label, look online also for the correct amount to add.
I add mine with the bottling sugar then rack on top.
Yup, I brew a Peach Wheat and use a 4oz container of Peach extract added to the bottling bucket along with the priming sugar and rack the beer on top. I brew 6 gallon batches and the 4oz is the perfect amount.
Boiled sugar goes into bottom of bottling bucket along with flavoring. Rack on top and stir when carboy's empty. I don't trust the whirlpool anymore, so I stir to make sure.
Be careful with cherry extract...they can make the beer taste like cough syrup. I've steered clear of them for that reason, and go with pure 100% dark cherry or tart cherry juice or concentrate you can find in the grocery store.
Image tart cherry would be better in a wheat. Maybe 1 full 32oz bottle.
Remember though, unlike extract, if you add cherry juice, you will get more fermentation activity due to the sugars being added so I wouldn't do it at bottling time unless you scale back on the priming sugar and are able to calculate exactly how much sugar the juice is replacing to prime the bottles.