You can set it in the dfp profiler but from what i remember you have to set it to a key that lfs doesn't mind being held down (like Ctrl) as when i set it to a letter ended up witht he annoying ringing/held down keep thing (still don't know what it is called lol)
This is from memory as i dont use the profiler anymore so might be a bit wrong - To set it just set TS to the button, and in profiler select the button you want to use as TS, click on it select keystroke, new keystroke, record then press key you want, press stop and then ok. I think then it is set but might not be so might have to then click on button and set the chosen key stroke as the key press.