If you mean board leash you will kill yourself with a leash on a board like that. DO NOT DO IT!!!
I have been trying to figure out a board leash for a raceboard, to only be used if the wind is so light you couldn't relaunch the kite, so the boardspeed would be less than 20 knots.
I've experimented in the past with flyreel leashes and it might be possible to get them to work with 30' or more of floating 3mm yellow polyethylene line.
I've also investigated the Flexi Maxi 8M dog leash and if a company like Oceanus built a reel leash like that with a floating yellow polyethylene strap, it would maybe wind in the board for the first few feet but after that you would be pulling in the rest by hand as the reel packed it neatly away.
However there is no 8M reel system with a stainless steel spring, so I dont plan to use a leash with the raceboard until I can check a system like that out. As I said, I only see a purpose for it if the wind is dropping so low that you couldn't relaunch the kite and want to paddle in on the board.
You dont want raceboard fins coming back at you guided by a leash, but I reckon 30' should be enough below 20 knots for it to be unlikely reach you if you keep your body near the surface.