I think that's an ESC setting instead of a transmitter setting.

I tried it when I built the t-rex, it lasted about a day, I absolutely couldn't stand that soft startup! I thought it was cool on startup, but the problem I didn't like was when you cut the throttle, it starts slowly again.. so, when I panick, cut the throttle a touch too much, I'm stuck doing a soft start to regain the heli... I didn't like that.. but try it for yourself.