[Richard_Lincoln]

Google "hill hold Audi retrofit" and look on the Audi sport net how to retrofit via vcds. You will lose launch control. Full coding and pass codes there.
Personally I had it on a previous car and hated it. Unable to park with gearbox in P with handbrake off without a juggle.
Easier just to pull switch, release footbrake and use throttle to automatically release. Simples.

 

[Richard_Lincoln]

Search "hold/assist via vcds audi" .
Audi sport net also has a thread on retrofitting button, wiring and coding. Some threads are not just applicable to the q5.
I found quite a few people looking for the same thing.
If you do not use the button method, purely vcds coding it states you lose launch control. Plus every time you stop the handbrake automatically applies. This was a pain in my last car as when I park (often for weeks at a time) I did not want to park with the handbrake on. Releasing it was a faff.
I'm not sure what problem you are trying to overcome that can't be overcome by pulling the switch. It may be you will just create another possibly worse problem...