You can use it... Start a race in 3d and hit esc to pause it, tab out and activate the option, the trainer crashes but the stamina "cheat" is working, tab back in and finish the race. After the race restart the game, or in the next 3d race it might crash at some point.
This works well for me however i can restart the trainer and use the other option aswell.
But the stamina doesnt work in time trials for me.