There's nothing unique about the bullhorns on the trek compared to most other base bars. I don't have pics, but I set them up on a couple of friend's bikes. So first, before we put any bar tape on, we just used a small piece of electrical tape on the wire right against the blip to stick it on the bar, and tried this in various places (inside of bar, underneath, further forward towards brake lever, further bac, etc) until we found the spot they liked the most for being able to hit the button. Then I put a piece of bar tape over it to see if they liked pressing it when underneath the tape, or whether they prefer the tape to go around it leaving it exposed (this was their choice).
Just experiment a bit. :)

I put mine where my pointer fingers tend to be on those grips. Otherwise I just tape them over with a single layer of electrical tape and then put bartape over them.