There is a hole under the throttle perch where you can fit a OEM friction throttle lock. There are two types that I have seen, one is a knurled screw, the other has a toggle. I had the knurled screw on mine but found it awkward and fiddly, hence the homemade one. I find mine is a lot easier to use, just activate on or off with my thumb. I also think it is a bit safer as you can either flick it off or just throttle back.
Regards Martin.