I found some specs online but It wouldn't work for Play.
I tried
const UInt8 noteOn[] = {0x90, 127}; and it didn't work.
Does anybody know what midi messages to HUI Pro Tools uses for play and stop?
There are 2 main Protocols out there for controlling DAWs, Logic Control and Mackie Control (HUI). Unfortunately both are close protocols. Only recently Apple added support for TouchOSC (iOS application) and the OSC protocol (Open Sound Control) in general for Logic Pro, hopefully Pro Tools to follow (maybe it already did and I'm not updated, you better check it out).
If you want to reverse engineer the record/stop buttons and you own some sort of Mackie Control device, I recommend using Midi Monitor or LC Xmu to monitor what data gets in. Not sure whats there for PC users, on my PC era I used my Pro Soundcard.
If you don't own some sort of controller and looking around the internet for the answer please notice that these protocols have many versions that each manufacture tweak a little bit. On the other hand, there are not that many options, you can try them ALL :)
Anyhow, I program an iOS application that controls Logic Pro without using LC or MC at all. I opened Logic's Key Command and set the Midi Listen button of the Start/Stop on, then sent some Midi Note from my iOS application to calibrate the button. It worked well, but was not intuitive to users so I decided to give up.