Hi. It seems this is the one key that can't be assigned to a controller button. I'm not sure exactly why but the way I've coded it, it seems to be half way to being a proper assignable control, like the pit speed limiter. And it's not just a "one press" key like the signal / indicator keys so you can't use the /press command. Maybe I didn't finish the job because I've run out of space in the controls screen, maybe because it's a bit laborious adding them, due to the way some of the controls code is written - it needs a small rewrite to make it easier to add.
I have made a note to look at this, and probably update the controls screen a bit, maybe with a scroll bar for the buttons - I'm not sure yet.