That thread will be on the top of the chrome shaft in the middle of the shocker
Make sure you try and tighten it with the car on its wheels and if need be stick some grips on it between the coils of the springs as high up as possible. You do risk ruining the shaft a bit but if done high enough it won't cause any leaks from the shocker!
There is a bearing in the top mount and a bush both of which could be worn so tightening that to FT might make bugger all difference anyway.